Imaging lens and imaging apparatus

This application is a Continuation of PCT International Application No. PCT/JP2013/004086 filed on Jul. 2, 2013, which claims priority under 35 U.S.C. §119(a) to Japanese Patent Application No. 2012-162665 filed on Jul. 23, 2012. Each of the above applications is hereby expressly incorporated by reference, in its entirety, into the present application.

1. Field of the Invention

The present invention relates to an imaging lens and an imaging apparatus, and more particularly to an imaging lens suitable for use in a vehicle mounted camera, a portable terminal camera, and a surveillance camera that utilize an image sensor, such as CCD's (Charge Coupled Device), CMOS's (Complementary Metal Oxide Semiconductor), and the like as well as to an imaging apparatus equipped with this imaging lens.

2. Description of the Related Art

In recent years, image sensors such as CCD's, CMOS's, and the like have achieved significant miniaturization and increased numbers of pixels. Accompanying these developments, as the bodies of imaging devices equipped with these image sensors also have achieved miniaturization, there is demand for imaging lenses to be mounted therein to be miniaturized in addition to having favorable optical performance. Meanwhile, there is demand for imaging lenses to be applied for use in a vehicle mounted camera, a surveillance camera, and the like to be miniaturized, be capable of being configured at low cost, and to have wider angles of view and higher performance.

Patent Documents 1 through 3 (Japanese Unexamined Patent Publication No. 2009-216858, Japanese Unexamined Patent Publication No. 2010-107531, and Japanese Unexamined Patent Publication No. 2010-072622) below propose imaging lenses of a six-lens configuration, in which a negative lens, a positive lens, a negative lens, a positive lens, a positive lens, and a negative lens are arranged in this order from the object side, as imaging lenses to be mounted on vehicle mounted cameras.

Requirements for imaging lenses to be mounted on vehicle mounted cameras, surveillance cameras, and the like are becoming rigorous year to year in such a manner that back focus is required to be secured in addition to achieving further miniaturization, lower costs, wider angles of view and higher performance.

The present invention has been developed in view of the foregoing circumstances. The object of the present invention is to provide an imaging lens which is capable of achieving miniaturization, low costs, a wider angle of view and high performance, and securing of back focus as well as an imaging apparatus equipped with this imaging lens.

The imaging lens of the present invention consists essentially of a first lens having a negative power, a second lens having a positive power, a third lens having a negative power, a fourth lens having a positive power, a fifth lens having a positive power, and a sixth lens having a negative power in this order from the object side, in which an aperture stop is disposed more toward the object side than the image-side surface of the fourth lens, and

the imaging lens satisfies conditional formula (4) below:

f 56 /f<− 6.4  (4),where

f: the focal length of the entire system, and f 56 : the combined focal length of the fifth lens and the sixth lens.

A second lens of the present invention consists essentially of a first lens having a negative power, a second lens having a positive power, a third lens having a negative power, a fourth lens having a positive power, a fifth lens having a positive power, and a sixth lens having a negative power in this order from the object side, in which the materials of the first lens and the second lens are glasses, the materials of the third lens through the sixth lens are plastics, an aperture stop is disposed more toward the object side than the image-side surface of the fourth lens, and conditional formula (5) below is satisfied:

f 34 /f 56<0.0  (5),where

f 34 : the combined focal length of the third lens and the fourth lens, f 56 : the combined focal length of the fifth lens and the sixth lens.

The imaging lens of the present invention consists essentially of six lenses. However, lenses substantially without any refractive power; optical elements other than lenses such stops, cover glasses, and the like; lens flanges; lens barrels; image sensors; and mechanical components such as image stabilization mechanisms may be included in addition to the six lenses.

Further, in the present invention, surface shapes of lenses, such as a convex surface, a concave surface, a planar surface, biconcave, meniscus, biconvex, plano-convex, plano-concave, and the like; and signs of the refractive powers of lenses, such as positive and negative, should be considered in a paraxial region if aspheric surfaces are included therein, unless otherwise noted. Moreover, in the present invention, the sign of the radius of curvature is positive in the case that a surface shape is convex on the object side, and negative in the case that the surface shape is convex on the image side. The expression “the center of the lens surface has a positive power” intends to mean that a value of a paraxial radius of curvature is such that the lens surface forms a convex surface. Further, the expression “the center of the lens surface has a negative power” intends to mean that a value of a paraxial radius of curvature is such that the lens surface forms a concave surface.

Note that it is preferable for the first and second imaging lenses of the present invention to satisfy conditional formulas (5-1) and (6) below:

−1.5 <f 34 /f 56<0.0  (5-1)

0.0 <f 34 /f   (6),where

f 34 : the combined focal length of the third lens and the fourth lens, and f 56 : the combined focal length of the fifth lens and the sixth lens.

Note that in the first imaging lens of the present invention, the materials of the third lens, the fourth lens, the fifth lens, and the sixth lens may be plastic.

Further, in the first imaging lens and the second imaging lens of the present invention, an aperture stop may be provided between the object-side surface of the second lens and the image-side surface of the fourth lens.

Further, in the first imaging lens and the second imaging lens of the present invention, the object-side surface of the fourth lens is an aspherical surface, both of the center and the edge of the effective diameter have a positive power, and the positive power at the edge of the effective diameter is weaker than that of the center.

The expression “having a positive power at the edge of the effective diameter” means having a convex shape at the edge of the effective diameter. The expression “having a negative power at the edge of the effective diameter” means having a concave shape at the edge of the effective diameter.

The expression “a shape in which a power at the edge of the effective diameter is weaker than that of the center” means “a shape in which a power at the edge of the effective diameter is weaker than that of the center” both in cases where the power is positive and where the power is negative.

In the first imaging lens and the second imaging lens of the present invention above, it is preferable for conditional formulas (1) through (3) and (7) through (12) to be satisfied. Note that preferably, the imaging lens may have a configuration, in which any one of conditional formulas (1) through (3) and (7) through (12) below is satisfied, or may have a configuration in which an arbitrary combination of two or more of the conditional formulas are satisfied.

2.38 <f 5 /f   (1)

−4.1 <R 1 /f< 0.0  (2)

0 <f 4 /f 5<0.45  (3)

2.0 <f 3456 /f   (7)

0.9 <νd 2 /νd 3  (8)

−2.5 <f 3 /f<− 0.5  (9)

−3.0 <f 3 /f 4<−0.2  (10)

0.2 <f 12 /f< 5.0  (11)

( Nd 1 +Nd 2 +Nd 3 +Nd 4 +Nd 5 +Nd 6)/6<1.70  (12),

where

f: the focal length of the entire system, R 1 : the radius of curvature of the object-side surface of the first lens, f 3 : the focal length of the third lens, f 4 : the focal length of the fourth lens, f 5 : the focal length of the fifth lens, f 12 : the combined focal length of the first lens and the second lens, f 3456 : the combined focal length of the third lens, the fourth lens, the fifth lens and the sixth lens, Nd 1 through Nd 6 : the refractive indices of the materials of the first lens through the sixth lens with respect to the d-line, νd 2 : the Abbe number of the material of the second lens with respect to the d-line, and νd 3 : the Abbe number of the material of the third lens with respect to the d-line.

The imaging apparatus of the present invention is mounted with at least any one of the first imaging lens and the second imaging lens of the present invention described above.

According to the first imaging lens of the present invention, a power arrangement, an arrangement of an aperture stop, and the like in the entire system are suitably set in a lens system constituted by the minimum number of lenses, i.e., six lenses, and conditional formula (4) is satisfied. This realizes an imaging lens which is capable of securing back focus while achieving miniaturization, low cost, and a wider angle of view. Further, an imaging lens having high optical performance, by which in the case that the fifth lens and the sixth lens are made of plastics, focus shift due to changes in temperature can be suppressed; various aberrations can be favorably corrected; and fine images can be obtained through the peripheral portions of the imaging area can be realized.

According to the second imaging lens of the present invention, a power arrangement, the materials of the lenses, an arrangement of an aperture stop, and the like in the entire system are suitably set in the lens system constituted by the minimum number of lenses, i.e., six lenses, and conditional formula (5) is satisfied. This realizes an imaging lens which is capable of achieving miniaturization, low cost, and a wider angle of view, and which has high optical performance, by which focus shift due to changes in temperature can be suppressed, various aberrations can be favorably corrected, and fine images can be obtained through the peripheral portions of the imaging area.

According to the imaging apparatus of the present invention, the imaging apparatus is provided with the imaging lens of the present invention. This enables the imaging apparatus to be configured in a small size and at low cost, to perform photography at a wide angle of view, and to obtain favorable images having high resolution.

FIG. 1 is a view illustrating a lens configuration and optical paths of an imaging lens of one embodiment of the present invention.

FIG. 2 is a view for explaining a surface shape and the like of the fourth lens.

FIG. 3 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 1 of the present invention.

FIG. 4 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 2 of the present invention.

FIG. 5 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 3 of the present invention.

FIG. 6 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 4 of the present invention.

FIG. 7 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 5 of the present invention.

FIG. 8 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 6 of the present invention.

FIG. 9 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 7 of the present invention.

FIG. 10 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 8 of the present invention.

FIG. 11 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 9 of the present invention.

FIG. 12 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 10 of the present invention.

FIG. 13 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 11 of the present invention.

FIG. 14 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 12 of the present invention.

FIG. 15 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 13 of the present invention.

FIG. 16 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 14 of the present invention.

FIG. 17 is a cross-sectional view illustrating the lens configuration of an imaging lens of Example 15 of the present invention.

A through D of FIG. 18 respectively illustrate aberration diagrams of the imaging lens of Example 1 of the present invention.

A through D of FIG. 19 respectively illustrate aberration diagrams of the imaging lens of Example 2 of the present invention.

A through D of FIG. 20 respectively illustrate aberration diagrams of the imaging lens of Example 3 of the present invention.

A through D of FIG. 21 respectively illustrate aberration diagrams of the imaging lens of Example 4 of the present invention.

A through D of FIG. 22 respectively illustrate aberration diagrams of the imaging lens of Example 5 of the present invention.

A through D of FIG. 23 respectively illustrate aberration diagrams of the imaging lens of Example 6 of the present invention.

A through D of FIG. 24 respectively illustrate aberration diagrams of the imaging lens of Example 7 of the present invention. A through D of FIG. 25 respectively illustrate aberration diagrams of the imaging lens of Example 8 of the present invention.

A through D of FIG. 26 respectively illustrate aberration diagrams of the imaging lens of Example 9 of the present invention.

A through D of FIG. 27 respectively illustrate aberration diagrams of the imaging lens of Example 10 of the present invention.

A through D of FIG. 28 respectively illustrate aberration diagrams of the imaging lens of Example 11 of the present invention.

A through D of FIG. 29 respectively illustrate aberration diagrams of the imaging lens of Example 12 of the present invention.

A through D of FIG. 30 respectively illustrate aberration diagrams of the imaging lens of Example 13 of the present invention.

A through D of FIG. 31 respectively illustrate aberration diagrams of the imaging lens of Example 14 of the present invention.

A through D of FIG. 32 respectively illustrate aberration diagrams of the imaging lens of Example 15 of the present invention.

FIG. 33 is a view for explaining an arrangement of a vehicle mounted imaging apparatus according to the embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[Embodiment of the Imaging Lens]

First, the imaging lens according to the embodiment of the present invention will be described referring to FIG. 1 . FIG. 1 is a view illustrating a lens configuration and optical paths of an imaging lens 1 according to the embodiment of the present invention. Note that the imaging lens 1 shown in FIG. 1 corresponds to an imaging lens according to Example 1 of the present invention to be described later.

In FIG. 1 , the left side of the figure is the object side, and the right side thereof is the image side. In addition, FIG. 1 also shows axial rays 2 from an object point at an infinite distance and off-axis rays 3 , 4 at a full angle of view 2 ω. Further, FIG. 1 shows an image sensor 5 disposed on the imaging plane Sim which includes an image point Pim of the imaging lens 1 , taking the case of applying the imaging lens 1 to an imaging apparatus into consideration. The image sensor 5 converts an optical image formed by the imaging lens 1 into an electric signal. A CCD image sensor, a CMOS image sensor, or the like may be employed as the image sensor, for example.

When the imaging lens 1 is applied to the imaging apparatus, it is preferable for a cover glass, a low-pass filter, an infrared cut filter, or the like to be provided according to the configurations of a camera on which the lens is mounted. FIG. 1 illustrates an example in which a plane parallel optical member PP that presumes such components is provided between the most-image-side lens and the image sensor 5 (the imaging plane Sim).

First, the configuration of the first embodiment of the present invention will be described. The imaging lens according to the first embodiment of the present invention includes a first lens L 1 having a negative power, a second lens L 2 having a positive power, a third lens L 3 having a negative power, a fourth lens L 4 having a positive power, a fifth lens L 5 having a positive power, and a sixth lens L 6 having a negative power in this order from the object side. In the example shown in FIG. 1 , an aperture stop St is disposed more object side than the image-side surface of the fourth lens L 4 , more particularly between the second lens L 2 and the third lens L 3 . Note that the aperture stop St shown in FIG. 1 does not necessarily represent the size or shape thereof, but represents the position thereof on the optical axis Z.

Further, the imaging lens of the first embodiment is configured to satisfy conditional formula (4) below:

f 56 /f<− 6.4  (4),where

f: the focal length of the entire system, and f 56 : the combined focal length of the fifth lens L 5 and the sixth lens L 6 .

Satisfying the upper limit defined by conditional formula (4) facilitates preventing the combined power of the fifth lens L 5 and the sixth lens L 6 from becoming too strong while making the combined power negative. In addition, correcting field curvature will be facilitated and suppressing the angles at which the rays enter the image sensor will be facilitated as well. Further, as suppressing the combined power of the fifth lens L 5 and the sixth lens L 6 will be facilitated, suppressing focus shift caused by changes in temperature will be facilitated in the case that the fifth lens L 5 and the sixth lens L 6 are made of a plastic.

Constituting the imaging lens of the second embodiment by the minimum number of lenses, i.e., six lenses enables low cost and reduction in the total length in the direction of the optical axis to be achieved. In addition, as the aperture stop St is disposed more toward the object side than the image-side surface of the fourth lens L 4 , reducing the diameter of each lens will be facilitated. Further, an imaging lens that has high optical performance, by which focus shift due to changes in temperature can be suppressed, various aberrations can be favorably corrected and fine images can be obtained through the peripheral portions of the imaging area, can be realized.

Next, the configuration of the second embodiment of the present invention will be described. The imaging lens according to the second embodiment of the present invention includes a first lens L 1 having a negative power, a second lens L 2 having a positive power, a third lens L 3 having a negative power, a fourth lens L 4 having a positive power, a fifth lens L 5 having a positive power and a sixth lens L 6 having a negative power in this order from the object. Further, in the example shown in FIG. 1 , an aperture stop St is disposed more toward the object side than the image-side surface of the fourth lens L 4 , more particularly between the second lens L 2 and the third lens L 3 . Further, the materials of the third lens L 3 through the sixth lens L 6 are plastics.

It is preferable for the imaging lens of the second embodiment to satisfy conditional formula (5) below:

f 34 /f 56<0.0  (5),where

f 34 : the combined focal length of the third lens L 3 and the fourth lens L 4 , and f 56 : the combined focal length of the fifth lens L 5 and the sixth lens L 6 .

Satisfying the upper limit defined by conditional formula (5) makes the value of conditional formula (5) negative. Therefore, one of f 34 and f 56 can be made positive and the other can be made negative. Accordingly, focus shift caused by changes in temperature can be easily suppressed by the positive and negative powers cancelling each other out when the temperature changes.

Configuring the imaging lens of the second embodiment to consist essentially of the minimum number of lenses, i.e., six lenses enables low cost and reduction in the total length in the direction of the optical axis to be achieved. In addition, as the aperture stop St is disposed more toward the object side than the image-side surface of the fourth lens L 4 , reducing the diameter of each lens will be facilitated. Further, as the materials of the third lens through the sixth lens are plastics, the lens system can be configured at low cost. Further, an imaging lens that has high optical performance, by which focus shift due to changes in temperature can be suppressed, various aberrations can be favorably corrected and fine images can be obtained through the peripheral portions of the imaging area, can be realized.

Next, preferable configurations of the imaging lenses according to the first and second embodiments above of the present invention and the advantageous effects thereof will be described. Note that preferably, the imaging lens may have any one of the configurations below, or may have an arbitrary combination of two or more of the configurations.

It is preferable for conditional formula (1) below to be satisfied:

2.38 <f 5 /f   (1),where

f: the focal length of the entire system, and f 5 : the focal length of the fifth lens L 5 .

Satisfying the lower limit defined by conditional formula (1) enables the power of the fifth lens L 5 to be suppressed, and facilitates securing back focus and correcting field curvature and comatic aberration. Securing a sufficient amount of back focus facilitates disposing various types of filters, a cover glass, or the like between the lens system and the image sensor as well as facilitating setting the exit pupil position far from the imaging plane. Thereby, suppressing the angles at which the peripheral rays enter the image sensor will be facilitated, resulting in facilitating suppression of shading.

It is preferable for conditional formula (2) to be satisfied:

−4.1 <R 1 /f< 0.0  (2),where

f: the focal length of the entire system, and R 1 : the radius of curvature of the object-side surface of the first lens L 1 .

Satisfying the upper limit defined by conditional formula (2) enables the object-side surface of the first lens L 1 to be made a concave surface, and facilitates increasing the power of the first lens L 1 , securing back focus, and reducing the size of the lens system in the radial direction. Satisfying the lower limit defined by conditional formula (2) facilitates preventing the radius of curvature of the object-side surface of the first lens L 1 from excessively decreasing, and facilitates suppressing distortion and preventing the total length from becoming too long.

It is preferable for conditional formula (3) to be satisfied:

0 <f 4 /f 5<0.45  (3),where

f 4 : the focal length of the fourth lens L 4 , and f 5 : the focal length of the fifth lens L 5 .

Satisfying the upper limit defined by conditional formula (3) facilitates causing the power of the fourth lens L 4 to be greater than that of the fifth lens L 5 . Increasing the power of the fourth lens L 4 facilitates correcting chromatic aberration between the third lens L 3 and the fourth lens L 4 and enables the power of the fifth lens L 5 to be suppressed. Further, securing back focus will be facilitated, and correcting field curvature and comatic aberration will be facilitated as well. Setting the value of the lower limit defined by conditional formula (3) to be 0 will cause the power of the fourth lens L 4 to become too strong or the power of the fifth lens L 5 to become too weak, resulting in correction of spherical aberration becoming difficult. Satisfying the lower limit defined by conditional formula (3) facilitates suppressing the power of the fourth lens L 4 or preventing the power of the fifth lens L 5 from becoming too weak, resulting in correction of spherical aberration being facilitated.

It is preferable for both conditional formula (5-1) and conditional formula (6) below to be satisfied:

−1.5 <f 34 /f 56<0.0  (5-1)

0.0 <f 34 /f   (6).

f 34 : the combined focal length of the third lens L 3 and the fourth lens L 4 , and f 56 : the combined focal length of the fifth lens L 5 and the sixth lens L 6 .

Satisfying conditional formula (6) enables the value of f 34 to be made positive. Satisfying the upper limit defined by conditional formula (5-1) and conditional formula (6) at the same time enables the values of f 34 and f 56 to be positive and negative in respective and enables focus shift due to changes in the temperature to be suppressed. Further, satisfying the lower limit defined by conditional formula (5-1) facilitates increasing the positive power of f 34 , thereby facilitating suppression of focus shift and correction of field curvature.

It is preferable for the materials of the third lens L 3 , the fourth lens L 4 , the fifth lens L 5 and the sixth lens L 6 to be plastic. This enables the lens to be configured at low cost.

It is preferable for conditional formula (7) below to be satisfied:

2.0 <f 3456 /f   (7),where

f 3456 : the combined focal length of the third lens L 3 , the fourth lens L 4 , the fifth lens L 5 and the sixth lens L 6 .

Satisfying the lower limit defined by conditional formula (7) facilitates preventing the combined power of the third lens L 3 through the sixth lens L 6 from becoming too strong as a positive power and facilitates securing back focus.

Further, it is preferable for the materials of the third lens L 3 , the fourth lens L 4 , the fifth lens L 5 and the sixth lens L 6 to be plastic and for conditional formula (7) above to be satisfied. This facilitates preventing the combined focal length of the plastic lenses from becoming too strong as a positive power and facilitates suppressing focus shift.

It is preferable for conditional formula (8) below to be satisfied:

0.9 <νd 2 /νd 3  (8),where

νd 2 : the Abbe number of the material of the second lens L 2 with respect to the d-line, and νd 3 : the Abbe number of the material of the third lens L 3 with respect to the d-line.

Decreasing the Abbe number of the third lens L 3 will be advantageous from the viewpoint of correcting longitudinal chromatic aberration. Satisfying the lower limit defined by conditional formula (8) facilitates balancing the ratios of the Abbe number of the materials of the second lens L 2 and the third lens L 3 and facilitates correcting chromatic aberration.

It is preferable for conditional formula (9) to be satisfied:

−2.5 <f 3 /f<− 0.5  (9),where

f: the focal length of the entire system, and f 3 : the focal length of the third lens L 3 .

Satisfying the lower limit defined by conditional formula (9) facilitates increasing the power of the third lens L 3 , thereby facilitating reducing longitudinal chromatic aberration. Satisfying the upper limit defined by conditional formula (9) facilitates suppressing the error sensitivity of the third lens L 3 , thereby facilitating manufacturing a lens which is resistant to axial displacement and the like.

It is preferable for the aperture stop St to be provided more toward the object side than the image-side surface of the fourth lens L 4 . Disposing the aperture stop St more toward the object side than the fourth lens L 4 facilitates reducing the diameters of the lenses. It is preferable for the aperture stop St to be disposed between the first lens L 1 and the second lens L 2 . This facilitates miniaturization of the first lens L 1 . For example, when the imaging lens is used as a vehicle mounted camera, there is demand for a lens surface which is exposed outside to be made small so as to improve the appearance of the car. Disposing the aperture stop St between the first lens L 1 and the second lens L 2 enables the lens which is exposed outside to be reduced in the size and facilitates improving the appearance of the car.

It is preferable for the aperture stop St to be disposed between the second lens L 2 and the third lens L 3 or between the third lens L 3 and the fourth lens L 4 . This improves balance of the diameters of the lenses at the front and back of the aperture stop St and facilitates suppressing the maximum diameters of the lenses, resulting in facilitating miniaturization of the lens. Further, disposing the aperture stop St between the second lens 2 and the third lens L 3 facilitates suppressing the angles at which the rays enter the image sensor while keeping a good balance of the diameters of the lenses at the front and back of the aperture stop St, resulting in facilitating suppressing the occurrence of shading.

It is preferable for conditional formula (10) below to be satisfied:

−3.0 <f 3 /f 4<−0.2  (10),where

f 3 : the focal length of the third lens L 3 , and f 4 : the focal length of the fourth lens L 4 .

Satisfying the upper and lower limits defined by conditional formula (10) can make good power balance between the third lens L 3 and the fourth lens L 4 , thereby facilitating correction of chromatic aberration. Satisfying the upper limit defined by conditional formula (10) can prevents the power of the third lens L 3 from becoming too strong, resulting in correction of field curvature being facilitated, or can prevents the power of the fourth lens L 4 from becoming too weak, resulting in correction of spherical aberration and field curvature being facilitated. Satisfying the lower limit defined by conditional formula (10) can prevent the power of the third lens L 3 from becoming too weak, resulting in correction of longitudinal chromatic aberration being facilitated, or can prevent the power of the fourth lens L 4 from becoming too strong, resulting in correction of spherical aberration and securing of back focus being facilitated.

It is preferable for conditional formula (11) below to be satisfied:

0.2 <f 12 /f< 5.0  (11),where

f: the focal length of the entire system, and f 12 : the combined focal length of the first lens L 1 and the second lens L 2 .

Satisfying the upper limit defined by conditional formula (11) facilitates increasing the combined power of the first lens L 1 and the second lens L 2 , resulting in facilitating correction of spherical aberration and field curvature. Satisfying the lower limit defined by conditional formula (11) facilitates preventing the combined power of the first lens L 1 and the second lens L 2 from becoming too strong as a positive power, resulting in correction of field curvature being facilitated.

It is preferable for three or less of the refractive indices among the refractive indices Nd 1 through Nd 6 of the materials of the first lens L 1 through the sixth lens L 6 with respect to the d-line to exceed 1.8. A refractive index exceeding 1.8 will increase the cost for the material of the lens. Therefore, it is preferable for the number of the refractive indices which exceed 1.8 to be three or less, even more preferably two or less, and still more preferably one or less.

It is preferable for conditional formula (12) below to be satisfied:

( Nd 1 +Nd 2 +Nd 3 +Nd 4 +Nd 5 +Nd 6)/6<1.70  (12),where

Nd 1 through Nd 6 : the refractive indices of the materials of the first lens through the sixth lens with respect to the d-line.

Satisfying the upper limit defined by conditional formula (12) facilitates suppressing the refractive index of each lens, resulting in reduction of the cost for the material being facilitated.

It is preferable for the number of the refractive indices which exceed 1.8 among the refractive indices Nd 1 through Nd 6 of the materials of the first lens L 1 through the sixth lens L 6 with respect to the d-line to be three or less, and for conditional formula (12) to be satisfied. This facilitates suppressing the refractive index of each lens, thereby facilitating the reduction in the cost for the material.

It is preferable for conditional formula (13) below to be satisfied:

2.0 <L/f< 7.0  (13),where

L: the distance from the object-side surface of the first lens L 1 to the imaging plane (back focus corresponds to the air converted length), and f: the focal length of the entire system.

Setting the value of L/f to exceed the upper limit defined by conditional formula (13) will increase the total length of the lens, resulting in miniaturization becoming difficult. Setting the value of L/f to fall below the lower limit defined by conditional formula (13) will make it difficult to widen the angle of view or will make the total length too short, thereby each lens becoming thin. As the result, manufacturing the lens will be difficult or the cost will increase.

It is preferable for conditional formula (14) below to be satisfied:

0.3 <Bf/f< 1.5  (14),where

Bf: the distance from the image-side surface of the sixth lens L 6 to the imaging plane (the air converted length), and f: the focal length of the entire system.

Setting the value of Bf/f to exceed the upper limit defined by conditional formula (14) will increase the amount of back focus, resulting in the size of the lens system being increased. Setting the value of Bf/f to fall below the lower limit defined by conditional formula (14) will make back focus too short, resulting in making it difficult to dispose various types of filters, a cover glass, or the like between the lens system and the image sensor.

Note that regarding the distance L from the object-side surface of the first lens L1 to the imaging plane along the optical axis and the distance Bf from the image-side surface of the most-image-side lens (the sixth lens L 6 ) to the imaging plane along the optical axis, an air converted length will be applied for the distance between the most-image-side lens and the imaging plane (in the case that a cover glass or various types of filters are disposed therebetween, the length corresponding thereto will be calculated in terms of an air converted length).

It is preferable for conditional formula (15) below to be satisfied:

45.0<(ν d 2 +νd 4 +νd 5)/3  (15),

where

νd 2 : the Abbe number of the material of the second lens L 2 with respect to the d-line, νd 4 : the Abbe number of the material of the fourth lens L 4 with respect to the d-line, and νd 5 : the Abbe number of the material of the fifth lens L 5 with respect to the d-line.

Satisfying the lower limit defined by conditional formula (15) facilitates increasing the Abbe number of each lens, thereby facilitating correction of longitudinal chromatic aberration and lateral chromatic aberration.

Note that it is preferable for the conditional formulas below, in which upper limits are added to the conditional formulas above or the lower or upper limits are changed in the conditional formulas above to be further satisfied so as to improve the above advantageous effects. In addition, preferably, the conditional formulas to be described below, each of which is configured by combining a changed value of the lower limit and a changed value of the upper limit, may be satisfied. Preferred modifications of conditional formulas will be described below as an example, but the modifications of conditional formulas are not limited to those listed below and the changed values described below may be combined.

It is preferable for conditional formula (1) to be provided with an upper limit, and for the upper limit defined by conditional formula (1) to be 30.0. This facilitates preventing the power of the fifth lens L 5 from becoming too weak, thereby facilitating correction of spherical aberration. Further, it is preferable for the upper limit defined by conditional formula (1) to be 20.0, more preferably 17.0, and even more preferably 8.0 in order to further facilitate correction of spherical aberration. It is preferable for the lower limit defined by conditional formula (1) to be 2.5, more preferably 3.0, and even more preferably 3.2. As described above, it is more preferable for conditional formulas (1-1) through (1-3) below to be satisfied, for example:

2.5 <f 5 /f< 20.0  (1-1)

3.0 <f 5 /f< 17.0  (1-2)

3.2 <f 5 /f< 17.0  (1-3).

It is preferable for the upper limit defined by conditional formula (2) to be −1.0. This further facilitates increasing the power of the first lens L 1 , and facilitates securing back focus and reducing the size of the lens system in the radial direction. Note that it is preferable for the upper limit defined by conditional formula (2) to be −2.0, and more preferably −2.5. Further, it is preferable for the lower limit defined by conditional formula (2) to be −3.9, more preferably −3.8, and even more preferably −3.7. As described above, it is more preferable for conditional formulas (2-1) through (2-3) below to be satisfied, for example:

−3.9 <R 1 /f<− 1.0  (2-1)

−3.9 <R 1 /f<− 2.0  (2-2)

−3.8 <R 1 /f<− 2.5  (2-3).

It is preferable for the upper limit defined by conditional formula (3) to be 0.40, more preferably 0.35, and even more preferably 0.30. Further, it is preferable for the lower limit defined by conditional formula (3) to be 0.02. This facilitates suppressing the power of the fourth lens L 4 or preventing the power of the fifth lens L 5 from excessively decreasing, resulting in correction of spherical aberration being facilitated. Note that it is more preferable for the lower limit defined by conditional formula (3) to be 0.05. As described above, it is more preferable for conditional formulas (3-1) through (3-3) below to be satisfied, for example:

0.02 <f 4 /f 5<0.40  (3-1)

0.02 <f 4 /f 5<0.35  (3-2)

0.05 <f 4 /f 5<0.35  (3-3).

It is preferable for the upper limit defined by conditional formula (4) to be −7.0, more preferably −7.2, and even more preferably −7.5. It is preferable for conditional formula (4) to be provided with the lower limit and for the lower limit defined by conditional formula (4) to be −100.0. Thereby, correction of field curvature will be facilitated by preventing the combined power of the fifth lens L 5 and the sixth lens L 6 , which is negative, from excessively decreasing. Further, it is preferable for the lower limit defined by conditional formula (4) to be −50.0, more preferably −45.0, and even more preferably −20.0 in order to further facilitate correction of field curvature. As described above, it is more preferable for conditional formulas (4-1) through (4-3) below to be satisfied, for example:

f 56 /f<− 7.0  (4-1)

f 56 /f<− 7.2  (4-2)

−50.0 <f 56 /f<− 7.2  (4-3).

It is preferable for the upper limit defined by conditional formula (5) to be −0.02, more preferably −0.04, and even more preferably −0.06. In the case that conditional formula (6) is satisfied, it is preferable for conditional formula (5) to be provided with the lower limit and the lower limit defined by conditional formula (5) to be −1.5. This facilitates increasing the positive power of f 34 , thereby further facilitating suppressing focus shift and correcting filed curvature. Further, it is preferable for the lower limit defined by conditional formula (5) to be −1.2, more preferably −1.0, even more preferably −0.9, still more preferably −0.7, and even still more preferably −0.6 in order to facilitate suppression of focus shift and correction of field curvature. As described above, it is more preferable for conditional formulas (5-2) through (5-4) below to be satisfied, for example:

−0.9 <f 34 /f 56<0.0  (5-2)

−1.2 <f 34 /f 56<−0.05  (5-3)

−0.7 <f 34 /f 56<−0.02  (5-4)

It is preferable for conditional formula (7) to be provided with an upper limit and for the upper limit defined by conditional formula (7) to be 6.0. This enables the combined power of the third lens L 3 through the sixth lens L 6 to be prevented from excessively decreasing, and facilitates suppressing the angles at which the rays enter the imaging surface and correcting field curvature. It is preferable for the upper limit defined by conditional formula (7) to be 5.0, and more preferably 4.0. It is preferable for the lower limit defined by conditional formula (7) to be 2.5, more preferably 2.7, and even more preferably 2.8. As described above, it is more preferable for conditional formulas (7-1) through (7-3) below to be satisfied, for example:

2.5 <f 3456 /f< 6.0  (7-1)

2.5 <f 3456 /f< 5.0  (7-2)

2.7 <f 3456 /f< 4.0  (7-3).

It is preferable for conditional formula (8) to be provided with an upper limit and for the upper limit defined by conditional formula (8) to be 3.5. This causes the Abbe number of the third lens L 3 to excessively decrease or the Abbe number of the second lens L 2 to excessively increase, resulting in avoiding high cost becoming easy. Further, decreasing the Abbe number of the third lens L 3 will cause the refractive index of the third lens L 3 to increase and enable the power of the third lens L 3 to be prevented from excessively increasing, resulting in correction of field curvature being facilitated. It is preferable for the upper limit defined by conditional formula (8) to be 3.0, and more preferably 2.5. It is preferable for the lower limit defined by conditional formula (8) to be 1.0, more preferably 1.2, and even more preferably 1.5. As described above, it is more preferable for conditional formulas (8-1) through (8-3) below to be satisfied, for example:

1.0 <νd 2 /νd 3<3.5  (8-1)

1.2 <νd 2 /νd 3<3.0  (8-2)

1.5 <νd 2 /νd 3<2.5  (8-3).

It is preferable for the upper limit defined by conditional formula (9) to be −0.6, more preferably −0.7, and even more preferably −0.8. It is preferable for the lower limit defined by conditional formula (9) to be −2.2, more preferably −2.0, even more preferably −1.8, and still more preferably −1.7. As described above, it is more preferable for conditional formulas (9-1) through (9-3) below to be satisfied, for example:

−2.2 <f 3 /f<− 0.6  (9-1)

−2.0 <f 3 /f<− 0.7  (9-2)

−1.8 <f 3 /f<− 0.8  (9-3).

It is preferable for the upper limit defined by conditional formula (10) to be −0.5, more preferably −0.7, and even more preferably −0.8. It is preferable for the lower limit defined by conditional formula (10) to be −2.5, more preferably −2.0, even more preferably −1.8, and still more preferably −1.7. As described above, it is more preferable for conditional formulas (10-1) through (10-3) below to be satisfied, for example:

−2.5 <f 3 /f 4<−0.5  (10-1)

−2.0 <f 3 /f 4<−0.7  (10-2)

−1.8 <f 3 /f 4<−0.8  (10-3).

It is preferable for the upper limit defined by conditional formula (11) to be 4.0, more preferably 3.0, and even more preferably 2.0. It is preferable for the lower limit defined by conditional formula (11) to be 0.5, more preferably 0.8, and even more preferably 1.0. As described above, it is more preferable for conditional formulas (11-1) through (11-3) below to be satisfied, for example:

0.5 <f 12 /f< 4.0  (11-1)

0.8 <f 12 /f< 3.0  (11-2)

1.0 <f 12 /f< 2.0  (11-3).

It is preferable for the upper limit defined by conditional formula (12) to be 1.68, and more preferably 1.64. It is preferable for conditional formula (12) to be provided with the lower limit and for the lower limit defined by conditional formula (12) to be 1.50. This enables the refractive indices of the materials of the first lens L 1 through the sixth lens L 6 with respect to the d-line to be prevented from excessively decreasing and facilitates increasing the power of each lens, thereby facilitating miniaturization of the lens system. It is preferable for the lower limit defined by conditional formula (12) to be 1.55, and more preferably 1.57. As described above, it is more preferable for conditional formulas (12-1) through (12-3) below to be satisfied, for example:

1.50<( Nd 1 +Nd 2 +Nd 3 +Nd 4 +Nd 5 +Nd 6)/6<1.70  (12-1)

1.55<( Nd 1 +Nd 2 +Nd 3 +Nd 4 +Nd 5 +Nd 6)/6<1.68  (12-2)

1.57<( Nd 1 +Nd 2 +Nd 3 +Nd 4 +Nd 5 +Nd 6)/6<1.64  (12-3).

It is preferable for the upper limit defined by conditional formula (13) to be 6.0, more preferably 5.0, and even more preferably 4.5. It is preferable for the lower limit defined by conditional formula (13) to be 2.5, more preferably 2.8, and even more preferably 3.0. As described above, it is more preferable for conditional formulas (13-1) through (13-3) below to be satisfied, for example:

2.5 <L/f< 6.0  (13-1)

2.8 <L/f< 5.0  (13-2)

3.0 <L/f< 4.5  (13-3).

It is preferable for the lower limit defined by conditional formula (14) to be 0.5, more preferably 0.6, and even more preferably 0.65. It is preferable for the upper limit defined by conditional formula (14) to be 1.2, more preferably 1.0, and even more preferably 0.9. As described above, it is more preferable for conditional formulas (14-1) through (14-3) below to be satisfied, for example:

0.5 <Bf/f< 1.5  (14-1)

0.6 <Bf/f< 1.2  (14-2)

0.6 <Bf/f< 1.0  (14-3).

It is preferable for conditional formula (15) to be provided with an upper limit and for the upper limit to be 60.0. Thereby, preventing the Abbe numbers of the materials of the second lens L 2 , the fourth lens L 4 and the fifth lens L 5 with respect to the d-line from excessively increasing facilitates increase of the refractive indices of the materials and further increasing the power of each lens facilitates reduction of the diameters of the lenses. Further, it will be easy to prevent the cost for the glass materials from increasing due to an excessive increase in the Abbe numbers of the materials. It is preferable for the upper limit defined by conditional formula (15) to be 58.0, and more preferably 56.0. It is preferable for the lower limit defined by conditional formula (15) to be 47.0, more preferably 49.0, even more preferably 50.0, and still more preferably 51.0. As described above, it is more preferable for conditional formulas (15-1) through (15-3) below to be satisfied, for example:

47.0<(ν d 2 +νd 4 +νd 5)/3  (15-1)

49.0<(ν d 2 +νd 4 +νd 5)/3<60.0  (15-2)

51.0<(ν d 2 +νd 4 +νd 5)/3<58.0  (15-3).

It is preferable for the Abbe number of the material of the first lens L 1 with respect to the d-line to be greater than or equal to 40. This enables longitudinal chromatic aberration and lateral chromatic aberration to be corrected favorably. Further, it is more preferable for the Abbe number of the material of the first lens L 1 with respect to the d-line to be greater than or equal to 50, and even more preferably greater than or equal to 55.

It is preferable for the Abbe number of the material of the second lens L 2 with respect to the d-line to be greater than or equal to 25. This enables longitudinal chromatic aberration to be corrected favorably. Further, it is more preferable for the Abbe number of the material of the second lens L 2 with respect to the d-line to be greater than or equal to 35, and even more preferably greater than or equal to 40.

It is preferable for the Abbe number of the material of the third lens L 3 with respect to the d-line to be less than or equal to 35. This enables longitudinal chromatic aberration to be corrected favorably. Further, it is more preferable for the Abbe number of the material of the third lens L 3 with respect to the d-line to be less than or equal to 30, even more preferably less than or equal to 28, and still more preferably less than or equal to 26.

It is preferable for the Abbe number of the material of the fourth lens L 4 with respect to the d-line to be greater than or equal to 40. This enables longitudinal chromatic aberration and lateral chromatic aberration to be corrected favorably. Further, it is more preferable for the Abbe number of the material of the fourth lens L 4 with respect to the d-line to be greater than or equal to 50, and even more preferably greater than or equal to 55.

It is preferable for the Abbe number of the material of the fifth lens L 5 with respect to the d-line to be greater than or equal to 40. This enables longitudinal chromatic aberration and lateral chromatic aberration to be corrected favorably. Further, it is more preferable for the Abbe number of the material of the fifth lens L 5 with respect to the d-line to be greater than or equal to 50, and even more preferably greater than or equal to 52.

It is preferable for the Abbe number of the material of the sixth lens L 6 with respect to the d-line to be less than or equal to 32. This enables lateral chromatic aberration to be corrected favorably. Further, it is more preferable for the Abbe number of the material of the sixth lens L 6 with respect to the d-line to be less than or equal to 26, and even more preferably less than or equal to 25.

It is preferable for at least one side surface of the fourth lens L 4 to be aspherical. Configuring at least one side surface of the fourth lens L 4 to be aspherical facilitates correction of field curvature and spherical aberration, resulting in enabling favorable resolution to be obtained. It is more preferable for both surfaces of the fourth lens L 4 to be aspherical.

It is preferable for the object-side surface of the fourth lens L 4 to be aspherical. It is preferable for the object-side surface of the fourth lens L 4 to have a shape in which both of the center and the edge of the effective diameter have positive powers and the positive power at the edge of the effective diameter is weaker than that of the center. Configuring the fourth lens L 4 to have such a shape facilitates correction of spherical aberration and field curvature.

Note that “the effective diameter of a surface” refers to the diameter of a circle constituted by an outermost point in the radial direction (a point farthest from the optical axis) among points where all of the rays contributing to image formation intersect with lens surfaces, and the term “edge of the effective diameter” refers to this outermost point. Note that in systems which have rotation symmetry with respect to the optical axis, a graphic constituted by the above outermost point is a circle. However, in systems which do not have rotation symmetry, the graphic is not a circle. In such a case, the diameter of an equivalent circle may be the effective diameter.

Further, regarding a shape of an aspherical surface, when a certain point on a lens surface i (i is a symbol which represents the corresponding lens surface. For example, when the object-side surface of the fourth lens L 4 is represented by 8, the following description with respect to the object-side surface of the fourth lens L 4 can be understood with i as 8.) of each lens is designated as Xi and an intersection of the normal line on the point and the optical axis is designated as Pi; the length (|Xi−Pi|) of Xi−Pi is defined as the absolute value |RXi| of the radius of curvature on the point Xi and Pi is defined as the center of curvature at the point Xi. Further, an intersection of the i-th lens surface and the optical axis is designated as Qi. In this case, a power at a point Xi is defined depending on whether a point Pi is on the object side or the image side based on a point Qi as the reference. In the object-side surface, in the case that a point Pi is toward the image side than a point Qi, the power is defined as positive, whereas in the case that the point Pi is toward the object side than the Qi, the power is defined as negative. In the image-side surface, in the case that the point Pi is toward the object side than the point Qi, the power is defined as positive, whereas in the case that the point Pi is toward the image side than the point Qi, the power is defined as negative.

When the power in the center is compared to the power at the point Xi, the absolute value of the radius of curvature in the center (paraxial radius of curvature) is compared to the absolute value |RXi| of the radius of curvature at the point Xi. In the case that |RXi| is smaller than the absolute value of the paraxial radius of curvature, the power at the point Xi is greater than the power in the center. In contrast, in the case that |RXi| is greater than the absolute value of the paraxial radius of curvature, the power at the point Xi is weaker than the power in the center. The same applies to both the case that a surface has the positive power and the case that a surface has the negative power.

Here, referring to FIG. 2 , the shape of the object-side surface of the fourth lens L 4 above will be described. FIG. 2 illustrates an optical path diagram of the imaging lens 1 shown in FIG. 1 . In FIG. 2 , a point Q 8 is the center of the object-side surface of the fourth lens L 4 and an intersection of the object-side surface of the fourth lens L 4 and the optical axis Z. Further, in FIG. 2 , the point X 8 on the object-side surface of the fourth lens L 4 is at the edge of the effective diameter, and is an intersection of the outermost ray 6 , which is included in off-axis rays 3 , and the object-side surface of the fourth lens L 4 . In FIG. 2 , although the point X 8 is at the edge of the effective diameter, the same applies to other points because the point X 8 is an arbitrary point on the object-side surface of the fourth lens L 4 .

In this case, an intersection of the normal line at a point X 8 on the lens surface and the optical axis Z is defined as a point P 8 as shown in FIG. 2 , a line segment X 8 -P 8 connecting between a point X 8 and a point P 8 is defined as the radius of curvature RX 8 at the point X 8 , and the length |X8-P8| of the line segment X8-P8 is defined as the absolute value |RX8| of the radius of curvature RX8. That is, |X8-P8| is |RX8|. Further, the radius of curvature at the point Q 8 , i.e., the radius of curvature in the center of the object-side surface of the fourth lens L 4 is designated as R 8 , and the absolute value thereof is designated as |R8| (not shown in FIG. 2 ).

The expression “a shape in which both of the center and the edge of the effective diameter have positive powers and the positive power at the edge of the effective diameter is weaker than that of the center” of the object-side surface of the fourth lens L 4 refers to, when a point X 8 is the edge of the effective diameter, a convex shape in a paraxial region including a point Q 8 and a shape in which a point P 8 is more toward the image side than the point Q 8 and the absolute value |RX8| of the radius of curvature at a point X 8 is greater than the absolute value |R8| of the radius of curvature at the point Q 8 .

It is preferable for at least one side surface of the fifth lens L 5 to be an aspherical surface. Configuring at least one side surface of the fifth lens L 5 to be an aspherical surface facilitates correction of field curvature and spherical aberration, thereby enabling favorable resolution to be obtained. It is more preferable for both side surfaces of the fifth lens L 5 to be aspherical.

It is preferable for the object-side surface of the fifth lens L 5 to be an aspherical surface. It is preferable for the object-side surface of the fifth lens L 5 to have a shape in which both of the center and the edge of the effective diameter have positive powers and in which the positive power at the edge of the effective diameter is greater than that of the center. Alternatively, it is preferable for the object-side surface of the fifth lens L 5 to have a shape in which the center has a negative power and the edge of the effective diameter has a positive power. Configuring the fifth lens L 5 to have such a shape facilitates correction of field curvature.

The above shape of the object-side surface of the fifth lens L 5 can be understood as described below in the same manner as in the shape of the object-side surface of the fourth lens L 4 which was described by using FIG. 2 . In a cross section of the lens, when a point on the object-side surface of the fifth lens L 5 is X 10 and the intersection of the normal line at the point and the optical axis Z is a point P 10 ; a line segment X 10 -P 10 connecting between the point X 10 and the point P 10 is the radius of curvature at the point X 10 , the length |X 10 -P 10 | of the line segment connecting between the point X 10 and the point P 10 is the absolute value |RX 10 | of the radius of curvature at the point X 10 . That is, |X 10 -P 10 | is |RX 10 |. Further, the intersection of the object-side surface of the fifth lens L 5 and the optical axis Z, i.e., the center of the object-side surface of the fifth lens L 5 is a point Q 10 . The absolute value of the radius of curvature at the point Q 10 is |R 10 |.

The expression “a shape in which both of the center and the edge of the effective diameter have positive powers and in which the positive power at the edge of the effective diameter is greater than that of the center” of the object-side surface of the fifth lens L 5 refers to, when a point X 10 is the edge of the effective diameter, a convex shape in the paraxial region including a point Q 10 and a shape in which a point P 10 is more toward the image side than the point Q 10 and the absolute value |RX 10 | of the radius of curvature at a point X 10 is smaller than the absolute value |R 10 | of the radius of curvature at the point Q 10 .

The expression “a shape in which the center has a negative power and the edge of the effective diameter has a positive power” of the object-side surface of the fifth lens L 5 refers to, when a point X 10 is the edge of the effective diameter, a concave shape in the paraxial region including a point Q 10 and a shape in which a point P 10 is more toward the image side than the point Q 10 .

It is preferable for the image-side surface of the fifth lens L 5 to be an aspherical surface. It is preferable for the image-side surface of the fifth lens L 5 to have a shape in which both of the center and the edge of the effective diameter have positive powers and the positive power at the edge of the effective diameter is weaker than that of the center. Configuring the fifth lens L 5 to have such a shape facilitates correction of field curvature.

The above shape of the image-side surface of the fifth lens L 5 can be understood as described below in the same manner as in the shape of the object-side surface of the fourth lens L 4 which was described by using FIG. 2 . In a cross section of the lens, when a point on the image-side surface of the fifth lens L 5 is X 11 and the intersection of the normal line at the point and the optical axis Z is a point P 11 ; a line segment X 11 -P 11 connecting between the point X 11 and the point P 11 is the radius of curvature at the point X 11 , the length |X 11 -P 11 | of the line segment connecting between the point X 11 and the point P 11 is the absolute value |RX 11 | of the radius of curvature at the point X 11 . That is, |X 11 -P 11 | is |RX 11 |. Further, the intersection of the image-side surface of the fifth lens L 5 and the optical axis Z, i.e., the center of the image-side surface of the fifth lens L 5 is a point Q 11 . The absolute value of the radius of curvature at the point Q 11 is |R 11 |.

The expression “a shape in which both of the center and the edge of the effective diameter have positive powers and in which the positive power at the edge of the effective diameter is weaker than that of the center” of the image-side surface of the fifth lens L 5 refers to, when a point X is the edge of the effective diameter, a convex shape in the paraxial region including a point Q 11 and a shape in which a point P 11 is more toward the object side than the point Q 11 and the absolute value |RX 11 | of the radius of curvature at a point X 11 is greater than the absolute value |R 11 | of the radius of curvature at the point Q 11 .

It is preferable for at least one side surface of the sixth lens L 6 to be an aspherical surface. Configuring at least one side surface of the sixth lens L 6 to be an aspherical surface facilitates correction of field curvature and spherical aberration, thereby enabling favorable resolution to be obtained. It is more preferable for both side surfaces of the sixth lens L 6 to be aspherical.

It is preferable for the object-side surface of the sixth lens L 6 to be an aspherical surface. It is preferable for the object-side surface of the sixth lens L 6 to have a shape in which both of the center and the edge of the effective diameter have negative powers and the negative power at the edge of the effective diameter is greater than that of the center. Configuring the sixth lens L 6 to have such a shape facilitates correction of field curvature.

The above shape of the object-side surface of the sixth lens L 6 can be understood as described below in the same manner as in the shape of the object-side surface of the fourth lens L 4 which was described by using FIG. 2 . In a cross section of the lens, when a point on the object-side surface of the sixth lens L 6 is X 12 and the intersection of the normal line at the point and the optical axis Z is a point P 12 ; a line segment X 12 -P 12 connecting between the point X 12 and the point P 12 is the radius of curvature at the point X 12 , the length |X 12 -P 12 | of the line segment connecting between the point X 12 and the point P 12 is the absolute value |RX 12 | of the radius of curvature at the point X 12 . That is, |X 12 -P 12 | is |RX 12 |. Further, the intersection of the object-side surface of the sixth lens L 6 and the optical axis Z, i.e., the center of the object-side surface of the sixth lens L 6 is a point Q 12 . The absolute value of the radius of curvature at the point Q 12 is |R 12 |.

The expression “a shape in which both of the center and the edge of the effective diameter have negative powers and in which the negative power at the edge of the effective diameter is greater than that of the center” of the object-side surface of the sixth lens L 6 refers to, when a point X 12 is the edge of the effective diameter, a concave shape in the paraxial region including the point Q 12 and a shape in which a point P 12 is more toward the object side than the point Q 12 and the absolute value |RX 12 | of the radius of curvature at a point X 12 is smaller than the absolute value |R 12 | of the radius of curvature at the point Q 12 .

It is preferable for at least one side surface of the third lens L 3 to be an aspherical surface. Configuring at least one side surface of the third lens L 3 to be an aspherical surface facilitates correction of field curvature and spherical aberration, thereby enabling favorable resolution to be obtained. It is more preferable for both side surfaces of the third lens L 3 to be aspherical.

It is preferable for the object-side surface of the third lens L 3 to be an aspherical surface. It is preferable for the object-side surface of the third lens L 3 to have a shape in which both of the center and the edge of the effective diameter have negative powers and the negative power at the edge of the effective diameter is weaker than that of the center. Configuring the object-side surface of the third lens L 3 to have such a shape facilitates correction of field curvature.

The above shape of the object-side surface of the third lens L 3 can be understood as described below in the same manner as in the shape of the object-side surface of the fourth lens L 4 which was described by using FIG. 2 . In a cross section of the lens, when a point on the object-side surface of the third lens L 3 is X 6 and the intersection of the normal line at the point and the optical axis Z is a point P 6 ; a line segment X 6 -P 6 connecting between the point X 6 and the point P 6 is the radius of curvature at the point X 6 , the length |X 6 -P 61 of the line segment connecting between the point X 6 and the point P 6 is the absolute value |RX 6 | of the radius of curvature at the point X 6 . That is, |X 6 -P 6 | is |RX 6 |. Further, the intersection of the object-side surface of the third lens L 3 and the optical axis Z, i.e., the center of the object-side surface of the third lens L 3 is a point Q 6 . The absolute value of the radius of curvature at the point Q 6 is |R 6 |.

The expression “a shape in which both of the center and the edge of the effective diameter have negative powers and in which the negative power at the edge of the effective diameter is weaker than that of the center” of the object-side surface of the third lens L 3 refers to, when a point X 6 is the edge of the effective diameter, a concave shape in the paraxial region including the point Q 6 and a shape in which a point P 6 is more toward the object side than the point Q 6 and the absolute value |RX 6 | of the radius of curvature at a point X 6 is greater than the absolute value |R 6 | of the radius of curvature at the point Q 6 .

It is preferable for the image-side surface of the third lens L 3 to be an aspherical surface. It is preferable for the image-side surface of the third lens L 3 to have a shape in which both of the center and the edge of the effective diameter have negative powers and in which the negative power at the edge of the effective diameter is greater than that of the center. Configuring the third lens L 3 to have such a shape facilitates correction of chromatic aberration.

The above shape of the image-side surface of the third lens L 3 can be understood as described below in the same manner as in the shape of the object-side surface of the fourth lens L 4 which was described by using FIG. 2 . In a cross section of the lens, when a point on the image-side surface of the third lens L 3 is X 7 and the intersection of the normal line at the point and the optical axis Z is a point P 7 ; a line segment X 7 -P 7 connecting between the point X 7 and the point P 7 is the radius of curvature at the point X 7 , the length |X 7 -P 7 | of the line segment connecting between the point X 7 and the point P 7 is the absolute value |RX 7 | of the radius of curvature at the point X 7 . That is, |X 7 -P 7 | is |RX 7 |. Further, the intersection of the image-side surface of the third lens L 3 and the optical axis Z, i.e., the center of the image-side surface of the third lens L 3 is a point Q 7 . The absolute value of the radius of curvature at the point Q 7 is |R 7 |.

The expression “a shape in which both of the center and the edge of the effective diameter have negative powers and in which the negative power at the edge of the effective diameter is greater than that of the center” of the image-side surface of the third lens L 3 means, when a point X 7 is the edge of the effective diameter, a concave shape in the paraxial region including the point Q 7 and a shape in which a point P 7 is more toward the image side than the point Q 7 and the absolute value |RX 7 | of the radius of curvature at a point X 7 is smaller than the absolute value |R 7 | of the radius of curvature at the point Q 7 .

The image-side surface of the third lens L 3 may have a shape in which both of the center and the edge of the effective diameter have negative powers and in which the negative power at the edge of the effective diameter is weaker than that of the center. Configuring the third lens L 3 to have such a shape facilitates correction of field curvature.

The expression “a shape in which both of the center and the edge of the effective diameter have negative powers and in which the negative power at the edge of the effective diameter is weaker than that of the center” of the image-side surface of the third lens L 3 refers to, when a point X 7 is the edge of the effective diameter, a concave shape in the paraxial region including the point Q 7 and a shape in which a point P 7 is more toward the image side than the point Q 7 and the absolute value |RX 7 | of the radius of curvature at a point X 7 is greater than the absolute value |R 7 | of the radius of curvature at the point Q 7 .

It is preferable for the first lens L 1 to be a biconcave lens. This enables the negative power of the first lens L 1 to increase, which is advantageous from the viewpoint of widening the angle of view and which facilitates securing a long back focus.

It is preferable for the second lens L 2 to be a biconvex lens. This enables the power of the second lens L 2 to increase and the combined powers of the first lens L 1 and the second lens L 2 to be well balanced even in the case that the power of the first lens L 1 is caused to be increased. Thereby, correction of comatic aberration and field curvature will be facilitated.

It is preferable for the third lens L 3 to be a biconcave lens. This enables the power of the third lens L 3 to increase, thereby facilitating correction of longitudinal chromatic aberration and lateral chromatic aberration.

It is preferable for the fourth lens L 4 to be a biconvex lens. This enables the power of the fourth lens L 4 to increase, thereby facilitating correction of chromatic aberration between the fourth lens L 4 and the third lens L 3 .

It is preferable for the fifth lens L 5 to have a convex surface toward the image side. This facilitates correction of field curvature.

The fifth lens L 5 may be a biconvex lens. This facilitates increasing the power of the fifth lens L 5 , thereby facilitating correction of spherical aberration.

The fifth lens L 5 may be a meniscus lens with a convex surface toward the image side. This facilitates correction of field curvature.

It is preferable for the sixth lens L 6 to be a lens with a concave surface toward the object side. This facilitates increasing the power of the sixth lens L 6 , thereby facilitating correction of chromatic aberration between the sixth lens L 6 and the fifth lens L 5 .

It is preferable for the sixth lens L 6 to be a meniscus lens with a concave surface toward the object side or to be a plano-concave lens with a concave surface toward the object side. This enables lateral chromatic aberration and field curvature to be corrected favorably. Telecentricity can be improved compared to the case that the sixth lens L 6 is a biconcave lens.

It is preferable for the object-side surface of the first lens L 1 to have a concave surface toward the object side. This facilitates increasing the power of the first lens L 1 , which is advantageous from the viewpoint of widening the angle of view, and facilitates reducing the diameter of the first lens L 1 , which is advantageous from the viewpoint of miniaturization.

It is preferable for the image-side surface of the first lens L 1 to be a concave surface. This facilitates increasing the power of the first lens L 1 , which is advantageous from the viewpoint of widening the angle of view.

It is preferable for the object-side surface of the second lens L 2 to be a convex surface. This facilitates increasing the power of the second lens L 2 , thereby facilitating correction of field curvature.

It is preferable for the image-side surface of the second lens L 2 to be a convex surface. This facilitates increasing the power of the second lens L 2 , thereby facilitating correction of field curvature.

It is preferable for the object-side surface of the third lens L 3 to be a concave surface. This facilitates increasing the power of the third lens L 3 , thereby facilitating correction of longitudinal chromatic aberration.

It is preferable for the image-side surface of the third lens L 3 to be a concave surface. This facilitates increasing the power of the third lens L 3 , thereby facilitating correction of longitudinal chromatic aberration.

It is preferable for the object-side surface of the fourth lens L 4 to be a convex surface. This facilitates increasing the power of the fourth lens L 4 , thereby facilitating correction of longitudinal chromatic aberration.

It is preferable for the image-side surface of the fourth lens L 4 to be a convex surface. This facilitates increasing the power of the fourth lens L 4 , thereby facilitating correction of longitudinal chromatic aberration.

It is preferable for the object-side surface of the fifth lens L 5 to be a convex surface. This facilitates correction of spherical aberration.

The object-side surface of the fifth lens L 5 may be a concave surface. This facilitates correction of field curvature.

It is preferable for the image-side surface of the fifth lens L 5 to be a convex surface. This facilitates correction of spherical aberration and field curvature.

It is preferable for the object-side surface of the sixth lens L 6 to be a concave surface. This facilitates correction of field curvature and lateral chromatic aberration.

It is preferable for the image-side surface of the sixth lens L 6 to be a planar surface or a convex surface. Configuring the image-side surface of the sixth lens L 6 to be a planar surface or a convex surface facilitates correction of field curvature.

The image-side surface of the sixth lens L 6 may be a concave surface.

It is preferable for the material of the first lens L 1 to be a glass. For example, when the imaging lens is used in severe environments as vehicle mounted cameras, surveillance cameras, and the like, there is demand for the first lens L 1 disposed on the most-object side to be made of a material which is resistant to surface deterioration caused by wind and rain, changes in temperature due to direct sunlight, and chemical agents such as oil, a detergent, and the like, i.e., a material which has high water resistance, weather resistance, acid resistance, chemical resistance, and the like. Further, there is demand for the first lens L 1 to be made of a material which is hard and not likely to break. Configuring the material to be a glass enables these demands to be satisfied. Alternatively, the material for the first lens L 1 may be a transparent ceramic.

Note that a protection means for improving the strength, scratch resistance, and chemical resistance may be provided on the object-side surface of the first lens L 1 . In such a case, the material of the first lens L 1 may be plastic. Such protection means may be a hard coat or a water-repelling coat.

In the imaging lens of the first embodiment, it is preferable for all the lenses to be formed of glass in order to manufacture an optical system which has superior environmental resistance. When the imaging lens is applied for use as a lens for a surveillance camera or a lens for a vehicle mounted camera, there is a possibility for the imaging lens to be used under various conditions such as a wide temperature range from a high temperature to a low temperature, high humidity, and the like. Therefore, it is preferable for all the lenses to be formed of glass in order to manufacture the optical system which is resistant to these conditions.

It is preferable for the material of the second lens L 2 to be a glass. Configuring the second lens L 2 to be formed of a glass facilitates using the material with high refractive index, thereby facilitating increase in the power of the second lens L 2 . As the result, correction of field curvature will become easy. Further, when plastics are applied for use in the third lens L 3 through the sixth lens L 6 , employing a glass for the second lens L 2 which is a convex lens facilitate suppressing focus shift due to changes in temperature.

In the imaging lens of the first embodiment, it is preferable for the materials of any one or a plurality of arbitrary combinations of the third lens L 3 through the sixth lens L 6 to be plastic. Configuring the materials to be plastic facilitates reduction in the cost and the weight of the lens system and enables aspherical surface shapes to be manufactured accurately and inexpensively, resulting in correction of spherical aberration and field curvature becoming possible.

It is preferable for the imaging lens to include a plastic lens having a positive power and a plastic lens having a negative power in order to manufacture the lens system which is resistant to changes in temperature. In general, plastic lenses have characteristics which vary significantly due to changes in temperature, which causes focus shift to occur. However, configuring the lens system to include the plastic lens having a positive power and the plastic lens having a negative power causes changes in the power to be cancelled out, thereby enabling deterioration in performance to be minimized.

Accordingly, in the imaging lens of the first embodiment, it is preferable for one of the third lens L 3 and the sixth lens L 6 and for one of the fourth lens L 4 and the fifth lens L 5 to be plastic. It is more preferable for all of the third lens L 3 through the sixth lens L 6 to be plastic lenses from the viewpoint of cost down.

Note that it is preferable for the number of the plastic lenses having positive powers and the number of the plastic lenses having negative powers to agree. This facilitates balancing the positive power with the negative power of the plastic lenses, thereby making it easy to suppress focus shift due to the changes in temperature. Note that if the positive power and the negative power are well balanced, the number of the plastic lenses having positive powers and the number of the negative lenses having negative powers do not have to agree.

In the imaging lens of the first embodiment, all of the four lenses, the third lens L 3 through the sixth lens L 6 may be plastic lenses, or only a pair of any of the lenses among these lenses may be plastic lenses. Alternatively, other lenses may be plastic lenses instead of the third lens L 3 through the sixth lens L 6 . For example, the second lens L 2 may be a plastic lens instead of the fourth lens L 4 . Further, the first lens L 1 may be a plastic lens instead of the sixth lens L 6 .

Acrylic, a polyolefin-based material, a polycarbonate-based material, an epoxy resin, PET (Polyethylene terephthalate), PES (Poly Ether Sulphone), a polycarbonate, and the like can be employed as the material of the plastic, for example.

It is preferable for the material of the third lens L 3 to be a polycarbonate-based material. This enables the Abbe number thereof to be decreased, thereby facilitating correction of longitudinal chromatic aberration and lateral chromatic aberration.

It is preferable for the material of the fourth lens L 4 to be a polyolefin-based material. This enables the Abbe number thereof to be increased and facilitates suppressing birefringence of the material, resulting in facilitating obtainment of a favorable resolution.

It is preferable for the material of the fifth lens L 5 to be a polyolefin-based material. This enables the Abbe number thereof to be increased and facilitates suppressing birefringence of the material, resulting in facilitating obtainment of a favorable resolution.

It is preferable for the material of the sixth lens L 6 to be a polycarbonate-based material. This enables the Abbe number thereof to be decreased, thereby facilitating correction of longitudinal chromatic aberration and lateral chromatic aberration.

It is preferable for the center thickness of the first lens L 1 to be greater than or equal to 0.8 mm. This enables a hard lens to be manufactured, thereby enabling a lens, which is resistant to various impacts, to be manufactured.

Note that a filter which cuts blue light from ultraviolet light or an IR (InfraRed) cutting filter which cuts infrared light may be provided between the lens system and the image sensor 5 according to the application of the imaging lens 1 . A coating which has the same characteristics as those of the filters above may be applied onto the lens surface. Alternatively, materials which absorb ultraviolet light, blue light, infrared light, and the like may be applied as the materials of any of the lenses.

FIG. 1 shows an example in which an optical member PP that presumes various types of filters, and the like is disposed between the lens system and the image sensor 5 , but these various types of filters may be disposed between the respective lenses, instead. Alternatively, a coating, which exhibits the same effects as the various types of filters, may be applied onto the lens surfaces of any of the lenses included in the imaging lens.

Note that there is a possibility that the rays which pass the outside of the effective diameters between the respective lenses will become stray light and reach the imaging plane, resulting in turning to ghosts. Accordingly, it is preferable for a light cutting means for shielding the stray light to be provided as necessary. As this light cutting means, an opaque paint may be applied onto portions of the outside of the effective diameters of the lenses, or an opaque plate may be provided therein, for example. Alternatively, opaque plates may be provided as the light cutting means on optical paths of the rays which become stray light. Alternatively, something like a hood for shielding stray light may be disposed more toward the object side than the most-object-side lens. As one example, FIG. 1 shows an example in which light cutting means 11 , 12 are provided at the exterior of the effective diameter of the image-side surface of each of the first lens L 1 and the third lens L 3 . Note that the positions in which the light cutting means are provided are not limited to the example shown in FIG. 1 , and the light cutting means may be provided on other lenses or between the lenses.

Further, members such as a stop, and the like which shields peripheral rays may be disposed between the respective lenses within a range in which no actual problems for the ratio of the amount of peripheral rays will arise. The peripheral rays are rays which pass through peripheral portions of an entrance pupil in the optical system among the rays emitted from object points outside of the optical axis Z. Disposing the member which shields the peripheral rays in such a manner enables image quality of the peripheral portions of the image formation region to be improved. Further, shielding the light which generates ghosts by the member enables ghosts to be reduced.

Further, it is preferable for the lens system to be constituted only by six lenses which are a first lens L 1 , a second lens L 2 , a third lens L 3 , a fourth lens L 4 , a fifth lens L 5 and the sixth lens L 6 . Constituting the lens system only by six lenses enables the lens system to be produced at low cost.

The imaging apparatus according to the present embodiment is provided with the imaging lens according to the present embodiment. Accordingly, the imaging apparatus can be configured in a small size and at low cost, have a sufficient wide angle of view, and obtain favorable images with high resolution by using the image sensor.

Note that images captured by the imaging apparatus provided with the imaging lens according to the first and second embodiments may be displayed on mobile phones. For example, there is a case that the imaging apparatus provided with the imaging lens of the present embodiment is mounted on a car as a vehicle mounted camera, the vehicle mounted camera captures images behind and around the car, and then the captured images is displayed on a display device. In such a case, in the cars mounted with a car navigation system, the captured images can be displayed on the display device of the car navigation system. However, in the case that the car navigation system is not mounted on the cars, dedicated display devices such as a liquid crystal display, and the like are required to be installed in the cars. However, display devices are expensive. Meanwhile, the recent mobile phones are equipped with display devices having high performance which enables viewing moving pictures and web sites. Using mobile phones as the display devices intended for vehicle mounted cameras makes it unnecessary to load dedicated display devices on cars which are not equipped with a car navigation system. As the result, vehicle mounted cameras can be mounted on cars at low cost.

Here, the images captured by the vehicle mounted camera may be wire-transmitted to a mobile phone via a cable, and the like or may be wirelessly transmitted to a mobile phone via infrared communication, and the like. Further, when the car's gear is set to reverse or a turn signal is activated, the images captured by the vehicle mounted camera may be automatically displayed on the display device of the mobile phone by coordinating the operating condition of the mobile phone with that of the car.

Note that the display device for displaying images captured by the vehicle mounted camera is not limited to a mobile phone, and may be such a portable data terminal as a PDA, and the like, a compact personal computer, or a laptop car navigation system.

Further, a mobile phone equipped with the imaging lens of the present invention may be fixed to a car to be used as a vehicle mounted camera. Recent smart phones have processing capability which is equivalent to those of PC's. Accordingly, the cameras for the mobile phones can be employed in the same manner as vehicle mounted cameras, for example by fixing a mobile phone to a dashboard, and the like in the car, and directing the camera forward. Note that a function for recognizing white lines and road signs and giving a caution may be included as an application for the smart phone. Further, the mobile phone may be a system which executes warnings when dozing and looking-aside are found by directing the camera towards a driver. Further, the mobile phone may be a part of the system that performs a steering wheel operation by coordinating with the car. There is demand for vehicle mounted camera to be resistant to severe environments because cars are left in high temperature environments and low temperature environments. When the imaging lens of the present invention is mounted on mobile phones, the mobile phones will be carried with drivers out of the cars except while driving. Accordingly, the imaging lens can be made less resistant to the environment; thereby a vehicle mounted system can be introduced at low cost.

Numerical Example of the Imaging Lens

Next, the Numerical Example of the imaging lens of the present invention will be described. The cross sectional views of the imaging lenses of the Examples 1 through 15 are shown in FIGS. 3 through 17 . In FIGS. 3 through 17 , the left side is the object side, and the right side is the image side. An aperture stop St, an optical member PP and an image sensor 5 disposed on an imaging plane Sim are also shown in the same manner as in FIG. 1 . An aperture stop St of each of the Figures does not necessarily represent the shape or size thereof, but the position thereof on the optical axis Z. In each Example, symbols Ri, Di (i=1, 2, 3, . . . ) of the cross sectional views of the lens respectively correspond to Ri, Di of the lens data to be described below.

Tables 1 through 15 show lens data of the imaging lenses of Examples 1 through 15. In each Table, A shows basic lens data, B shows various types of data and C shows aspherical surface data.

In basic lens data, the column of Si shows the i-th (i=1, 2, 3, . . . ) surface number, the value of i sequentially increasing from the object-side surface of the constituent element at the most object side, which is designated as 1, toward the image side. The column Ri shows the radii of curvature of the i-th surface, and the column Di shows the distances between i-th surfaces and i+1st surfaces along the optical axis Z. Note that the sign of the radius of curvature is positive in the case that a surface shape has a convex surface toward the object side, and negative in the case that a surface shape has a convex toward the image side. Further, the column Ndj shows the refractive indices of j-th (j=1, 2, 3, . . . ) optical elements with respect to the d-line (wavelength: 587.6 nm), the value of j sequentially increasing from the constituent element at the most object side, which is designated as 1, toward the image side. The column νdj shows the Abbe numbers of j-th optical elements with respect to the d-line (wavelength: 587.6 nm). Note that the basic lens data also shows the aperture stop St and an optical member PP. The column of the surface number of a surface corresponding to the aperture stop St indicates the text (St).

In the basic lens data, the mark “*” is indicated at surface numbers of aspherical surfaces. Numerical values of paraxial radii of curvature (the radii of curvature of the center) are shown as the radii of curvature of aspherical surfaces. The aspherical surface data shows surface numbers of the aspherical surfaces and aspherical surface coefficients with respect to the aspherical surfaces. Note that “E−n” (n: integer) in each of the numerical values of the aspherical surface coefficients means “×10 −n ”, and “E+n” therein means “×10 n ”. The aspherical surface coefficients are the values of respective coefficients KA, RBm (m=3, 4, 5, . . . 20) in the aspherical surface formula below:

Zd=C·h 2 /{1+(1 −KA·C 2 ·h 2 ) 1/2 }+ΣRBm·h m

where,

Zd: the depth of an aspherical surface (the length of a perpendicular line drawn from a point on an aspherical surface with a height h to a plane perpendicular to the optical axis which contacts the peak of the aspherical surface) h: height (the distance from the optical axis to a lens surface) C: an inverse number of a paraxial radius of curvature KA, RBm: aspherical surface coefficients (m=3, 4, 5, . . . 20).

In the various data, L (in Air) is the distance (back focus corresponds to an air converted length) from the object-side surface of the first lens L 1 to the imaging plane Sim along the optical axis Z, Bf (in Air) is the distance (which corresponds to back focus, the air converted length) from the image-side surface of the most-image-side lens to the imaging plane Sim along the optical axis Z, f is the focal length of the entire system, f 1 is the focal length of the first lens L 1 , f 2 is the focal length of the second lens L 2 , f 3 is the focal length of the third lens L 3 , f 4 is the focal length of the fourth lens L 4 , f 5 is the focal length of the fifth lens L 5 , f 6 is the focal length of the sixth lens L 6 , f 12 is the combined focal length of the first lens L 1 and the second lens L 2 , f 34 is the combined focal length of the third lens L 3 and the fourth lens L 4 , f 56 is the combined focal length of the fifth lens L 5 and the sixth lens L 6 , f 3456 is the combined focal length of the third lens L 3 , the fourth lens L 4 , the fifth lens L 5 and the sixth lens L 6 .

Tables 16 and 17 collectively show values respectively corresponding to the conditional formulas (1) through (15) of each Example. Note that conditional formula (1) corresponds to f 5 /f, conditional formula (2) corresponds to R 1 /f, conditional formula (3) corresponds to f 4 /f 5 , conditional formula (4) corresponds to f 56 /f, conditional formula (5) corresponds to f 34 /f 56 , conditional formula (6) corresponds to f 34 /f, conditional formula (7) corresponds to f 3456 /f, conditional formula (8) corresponds to νd 2 /νd 3 , conditional formula (9) corresponds to f 3 /f, conditional formula (10) corresponds to f 3 /f 4 , conditional formula (11) corresponds to f 12 /f, conditional formula (12) corresponds to (Nd 1 +Nd 2 +Nd 3 +Nd 4 +Nd 5 +Nd 6 )/6, conditional formula (13) corresponds to L/f, conditional formula (14) corresponds to Bf/f, and conditional formula (15) corresponds to (νd 2 +νd 4 +νd 5 )/ 3 .

where,

L: the distance (back focus corresponds to the air converted length) from the peak of the object-side surface of the first lens L 1 to the imaging plane,

Bf: the distance (an air converted length) from the peak of the image-side surface of the sixth lens L 6 to the imaging plane,

R 1 : the radius of curvature of the object-side surface of the first lens L 1 ,

f: the focal length of the entire system,

f 1 : the focal length of the first lens L 1 ,

f 3 : the focal length of the third lens L 3 ,

f 4 : the focal length of the fourth lens L 4 ,

f 5 : the focal length of the fifth lens L 5 ,

f 12 : the combined focal length of the first lens L 1 and the second lens L 2 ,

f 34 : the combined focal length of the third lens L 3 and the fourth lens L 4 ,

f 56 : the combined focal length of the fifth lens L 5 and the sixth lens L 6 ,

f 3456 : the combined focal length of the third lens L 3 , the fourth lens L 4 , the fifth lens L 5 and the sixth lens L 6 ,

Nd 1 through Nd 6 : the refractive indices of the materials of the first lens L 1 through the sixth lens L 6 with respect to the d-line,

νd 2 : the Abbe number of the material of the second lens L 2 with respect to the d-line,

νd 3 : the Abbe number of the material of the third lens L 3 with respect to the d-line,

νd 4 : the Abbe number of the material of the fourth lens L 4 with respect to the d-line, and

νd 5 : the Abbe number of the material of the fifth lens L 5 with respect to the d-line.

Regarding the unit of each numerical value, mm is used as the unit of length, but this is only an example and other appropriate units may also be used, as optical systems are usable even when they are proportionally enlarged or miniaturized.

In the imaging lenses of Examples 1 through 15 above, the first lens L 1 and the second lens L 2 are glass spherical lenses, and the third lens L 3 through the sixth lens L 6 are plastic aspherical lenses. Note that basic lens data indicates the names of the materials of the first lens L 1 and the second lens L 2 . For example, S-BAL35 by Ohara Corp. is indicated as the material of the first lens L 1 of Examples 1 through 12 in the basic lens data. However, glass materials by other companies having the same characteristics may be employed instead. For example, BACD5 by HOYA Co., Ltd., K-SK5 by Sumita, H-ZK3 by Chengdu Guangming Co., and the like may be employed instead of S-BAL35 by Ohara Co. Further, S-LAH 58 by Ohara Co. is indicated as the material of the second lens L 2 of each of Examples 1 through 12. However, TAFD30 by HOYA Co., Ltd., K-LASFN17 by Sumita, and H-ZLAF68 by Chengdu Guangming Co. may be employed instead of S-LAH 58 by Ohara Co.

Further, BSC7 by HOYA Co., Ltd. is indicated as the material of the first lens L1 of Example 13. However, S-BSL7 by Ohara Corp., K-BK7 by Sumita, H-K9L by Chengdu Guangming Co., and N-BK7 by Schott may be employed instead of BSC7 by HOYA Co., Ltd. Further, S-LAH65 by Ohara Corp. is indicated as the material of the second lens L2 of Example 13. However, TAF3 by HOYA Co., Ltd., K-LASFN6 by Sumita, H-ZLAF50B by Chengdu Guangming Co., and the like may be employed instead of S-LAH65 by Ohara Corp.

Further, Ohara Corp. S-BSM10 is indicated as the material of the first lens L 1 of Example 14. However, E-BACD10 by HOYA Co., Ltd. and H-ZK10 by Chengdu Guangming Co. may be employed instead of S-BSM10 by Ohara Corp. Further, S-YGH51 by Ohara Corp. is indicated as the material of the second lens L 2 of Example 14 and the first lens L 1 of Example 15. However, TAC6 by HOYA Co., Ltd., K-LASKN1 by Sumita, H-LAK53A by Chengdu Guangming Co., and the like may be employed instead of S-YGH51 by Ohara Corp. Further, S-LAH55 by Ohara Corp. is indicated as the material of the second lens L 2 of Example 15. However, TAFD5F by HOYA Co., Ltd., K-LASFN8 by Sumita, H-ZLAF55A by Chengdu Guangming Co., and the like may be employed instead of S-LAH55 by Ohara Corp.

[Aberration Performance]

The respective aberration diagrams of the imaging lenses according to Examples 1 through 15 above are shown in A through D of FIG. 18 , A through D of FIG. 19 , A through D of FIG. 20 , A through D of FIG. 21 , A through D of FIG. 22 , A through D of FIG. 23 , A through D of FIG. 24 , A through D of FIG. 25 , A through D of FIG. 26 , A through D of FIG. 27 , A through D of FIG. 28 , A through D of FIG. 29 , A through D of FIG. 30 , A through D of FIG. 31 and A through D of FIG. 32 .

Here, the aberration diagram of Example 1 will be described as an example. The same applies to the aberration diagrams of the other Examples. A, B, C and D of FIG. 18 respectively show the aberration diagrams of spherical aberration, astigmatism, distortion, and lateral chromatic aberration of the imaging lens according to Example 1. F in each of spherical aberrations refers to an F value, ω in each of the other aberration diagrams refers to a half angle of view. Distortion diagrams show the amount of displacement from an ideal image height which is f×tan (φ) by using the focal length f of the entire system and an angle of view φ (which is a variable, 0≦φ≦ω). Each aberration diagram shows aberration with respect to the d-line (wavelength: 587.56 nm) as the reference wavelength. The spherical aberration diagram also shows aberrations with respect to the F-line (wavelength: 486.13), the C-line (wavelength: 656.27 nm), the s-line (wavelength: 852.11 nm) and aberration with respect to the offense against the sine condition (denoted as SNC). The lateral chromatic aberration diagram also shows aberrations with respect to the F-line, the C-line and the s-line. The types of lines in the lateral chromatic aberration diagram are the same as those in the spherical aberration diagram. Accordingly, redundant descriptions thereof will be omitted.

As can be found from the data described above, each of the imaging lenses of Examples 1 through 15 is constituted by the small number of lenses, i.e., six lenses; and can be produced in a small size and at low cost. The respective imaging lenses further have F numbers of between 1.8 and 2.0 which are small, and have high optical performance with each aberration corrected favorably. These imaging lenses can be suitably used for surveillance cameras, vehicle mounted cameras for photographing images in the front, side, and back of an automobile, and the like.

FIG. 33 shows the aspect of an automobile 100 on which the imaging apparatus provided with the imaging lens of the present embodiment is mounted, as a usage example. In FIG. 33 , the automobile 100 is provided with an outside-vehicle camera 101 for photographing a blind angle range on the side surface of the passenger's side thereof, an outside-vehicle camera 102 for photographing a blind angle range behind the automobile 100 , and an in-vehicle camera 103 , which is provided on the back of a room mirror, for photographing the same visual field range as the driver's. The outside-vehicle cameras 101 , 102 and the in-vehicle camera 103 correspond to the imaging apparatus according to the embodiment of the present invention, and are provided with the imaging lens according to the present embodiment of the present invention and an imaging element which converts an optical image formed by the imaging lens into an electric signal.

All the imaging lenses according to the Examples of the present invention have the advantageous points described above. Accordingly, the outside-vehicle cameras 101 , 102 and the in-vehicle camera 103 can be also configured in a small size and at low costs, have wider angles of view, and enables fine images to be obtained through the peripheral portions of the imaging area.

The present invention has been described with reference to the Embodiments and Examples. The present invention is not limited to the embodiments and the examples described above, and various modifications are possible. For example, values, such as the radius of curvature, the distances between surfaces, the refractive indices, the Abbe numbers of each lens element, and the like are not limited to the values in the numerical examples shown in the Tables, but may be other values.

Note that all of the lenses of the Examples above are constituted by homogeneous materials. However, gradient index lenses may be used as the lenses. Further, in some of the Examples above, the second lens L 2 through the sixth lens L 6 are constituted by diffractive lenses in which surfaces are made aspherical. A diffractive optical element may be formed on one surface or a plurality of surfaces.

The embodiment of the imaging apparatus was described with reference to the Figure of an example, in which the present invention is applied to a vehicle mounted camera. The present invention is not limited to this application and can be applied to portable terminal cameras, surveillance cameras, and the like, for example.