Imaging lens

The present invention relates to an imaging lens for forming an optical image on an imaging element such as a CCD sensor and a CMOS sensor. Particularly, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a cellular phone, a digital still camera, a portable information terminal, a security camera, an onboard camera, and a network camera.

Performances of a camera mounted in a cellular phone have been improved every year, and there is even a camera having optical performances comparable to those of a digital still camera. The resolution of an imaging element that used to be several hundred thousand pixels has been improved to several megapixels, and the resolution of a camera has been dramatically improved. An imaging lens for mounting in such camera needs to not only have a small size, but also have resolution comparable to that of a high-resolution imaging element, i.e., ability to satisfactorily correct aberrations.

Conventionally, in order to attain both sufficient optical performances and miniaturization, it is typical to adopt an imaging lens having two or three-lens configuration. However, optical performances required for such a high-resolution imaging element have been increased each year, so that it is difficult to sufficiently correct aberrations anymore. Further, it is difficult to meet those optical performance requirements with such a two- or three-lens configuration. For this reason, a four- or five-lens configuration has been studied and practically applied in recent years.

Among them, since a five-lens configuration has high flexibility in designing, it is expected to be a lens configuration to be applied to a next generation imaging lens. For example, an imaging lens described in Patent Reference includes a first lens that is convex on an object-side surface thereof and is positive; a second lens that has a shape of a negative meniscus lens directing a concave surface thereof to an image plane side; a third lens that has a shape of a positive meniscus lens directing a convex surface thereof to the image plane side; a fourth lens that has aspheric shapes on both surfaces thereof, is concave on an image plane-side surface thereof near an optical axis, and is negative; and a fifth lens that has aspheric shapes on both surfaces thereof and is positive or negative, arranged in this order from the object side. In addition, there are limitations in a lower limit of Abbe's number of the first lens and an upper limit of Abbe's number of the second and the fourth lenses. With the lens configuration like this, according to the imaging lens, it is possible to satisfactorily correct an axial chromatic aberration and a chromatic aberration of magnification.

Patent Reference: Japanese Patent Application Publication No. 2007-264180

According to the imaging lens described in Patent Reference, it is possible to obtain relatively satisfactory aberrations. However, since the total length of the lens system is long, it is difficult to attain both miniaturization of the imaging lens and satisfactory aberration correction. Here, such challenge of attaining both miniaturization and satisfactory aberration correction is not specific to the imaging lens for mounting in a cellular phone, but common among imaging lenses for mounting in relatively small cameras such as digital still cameras, portable information terminals, security cameras, onboard cameras, and network cameras.

In view of the above-described problems, an object of the present invention is to provide an imaging lens that can satisfactorily correct aberrations in spite of the small size thereof.

Further objects and advantages of the invention will be apparent from the following description of the invention.

In order to attain the objects described above, according to an aspect of the present invention, an imaging lens includes a first lens having positive refractive power; a second lens having negative refractive power; a third lens having positive refractive power; a fourth lens having negative refractive power; and a fifth lens having negative refractive power, arranged in the order from an object side to an image plane side. The first lens is formed in a shape so that a curvature radius of an object-side surface thereof is positive and the second lens is formed in a shape so that a curvature radius of an object-side surface thereof and a curvature radius of an image plane-side surface thereof are both positive. The third lens is formed in a shape so that a curvature radius of an object-side surface thereof is positive, and the fifth lens is formed in a shape so that a curvature radius of an object-side surface thereof and a curvature radius of an image plane-side surface thereof are both positive. According to the configuration, the imaging lens of the invention satisfies a condition that an Abbe's number of each lens from the first lens, and the third lens to the fifth lens is greater than 45 and an Abbe's number of the second lens is less than 35.

In order to enhance resolution of an imaging lens so as to be compatible with a high-resolution imaging element available in these years, it is necessary to satisfactorily correct aberrations, especially chromatic aberrations. According to a configuration of the imaging lens of the invention, the Abbe's number of the first lens is greater than 45 and the Abbe's number of the second lens is less than 35, and the first lens and the second lens are formed by combining a low-dispersion material and a high-dispersion material. In addition, each lens from the third lens to the fifth lens has the Abbe's number greater than 45 and is formed of a low-dispersion material. Therefore, a chromatic aberration occurred in the first lens is corrected by the second lens and at the same time, satisfactorily corrected through each lens from the third lens to the fifth lens.

Here, according to the imaging lens having the above-described configuration, preferably, each lens of the first lens and lenses from the third to the fifth lenses has the same Abbe's number and is made of the same material. With this configuration, it is possible to suitably attain productivity improvement and manufacturing cost reduction of the imaging lens. In addition, forming each lens from the first lens to the fifth lens from a plastic material, it is possible to further attain productivity improvement and manufacturing cost reduction.

When the first lens has a focal length f 1 , the second lens has a focal length f 2 , the third lens has a focal length f 3 , the fourth lens has a focal length f 4 , and the fifth lens has a focal length f 5 , the imaging lens having the above-described configuration preferably satisfies the following conditional expressions (1) and (2):

f1<f3 and |f2|<f3  (1)

f3<|f4| and |f2|<|f5|  (2)

When the imaging lens satisfies the conditional expressions (1) and (2), it is possible to satisfactorily correct aberrations while attaining miniaturization of the imaging lens. When the imaging lens satisfies the conditional expressions (1) and (2), the first lens and the second lens, which are arranged on the object side, have stronger refractive power than other three lenses. With the configuration, it is possible to shorten a total optical length of the imaging lens while securing certain angle of view, and it is possible to suitably attain miniaturization of the imaging lens. In addition, according to the invention, each lens from the third lens to the fifth lens has relatively weak refractive power. Aberrations occurred in the first lens and the second lens, which have strong refractive powers, are suitably corrected through each lens from the third lens to the fifth lens, which have weak refractive powers.

In the imaging lens having the above-described configuration, the fourth lens is preferably formed in a shape so that a curvature radius of an object-side surface thereof and a curvature radius of an image plane-side surface thereof are both negative.

As described above, the second lens is formed in a shape so that a curvature radius of the object-side surface thereof and a curvature radius of the image plane-side surface thereof are both positive, i.e. a shape of a meniscus lens directing a convex surface thereof to the object side. Furthermore, the fourth lens is formed in a shape so that a curvature radius of the object-side surface thereof and a curvature radius of the image plane-side surface thereof are both negative, i.e. a shape of a meniscus lens directing a concave surface thereof to the object side. Therefore, the second lens and the fourth lens are arranged disposing their concave surfaces to the third lens. With this configuration, aberrations occurred in the first lens are suitably corrected also by the negative-positive-negative refractive power arrangement of the lenses from the second to the fourth lenses and the shapes of the respective lenses of the second lens and the fourth lens.

When the whole lens system has a focal length f and the second lens has the focal length f 2 , the imaging lens having the above-described configuration preferably satisfies the following conditional expression (3):

−1.8<f2/f<−0.8  (3)

When the imaging lens satisfies the conditional expression (3), it is possible to restrain a chromatic aberration and a field curvature within preferred ranges. When the value exceeds the upper limit “−0.8”, since the second lens has strong refractive power in relative to the whole lens system, an axial chromatic aberration is excessively corrected (a focal position at a short wavelength moves towards the image plane side in relative to a focal position at a reference wavelength), and at the same time, an off-axis chromatic aberration of magnification is excessively corrected (an image-forming point at a short wavelength moves in a direction to be away from the optical axis in relative to an image-forming point at a reference wavelength). In addition, an image-forming surface curves to the image plane side and it is difficult to obtain satisfactory image-forming performance. On the other hand, when the value is below the lower limit “−1.8”, the second lens has weak refractive power in relative to the whole lens system, so that an axial chromatic aberration is insufficiently corrected (a focal position at a short wavelength moves towards the object side in relative to a focal position at a reference wavelength) and an off-axis chromatic aberration of magnification is insufficiently corrected (an image-forming point at a short wavelength moves in a direction to approach the optical axis in relative to an image-forming point at a reference wavelength). Moreover, an image-forming surface curves to the object side, so that it is difficult to obtain satisfactory image-forming performance also in this case.

When the first lens has the focal length f 1 and the second lens has the focal length f 2 , the imaging lens having the above-described configuration preferably satisfies the following conditional expression (4):

−1.0<f1/f2<−0.4  (4)

When the imaging lens satisfies the conditional expression (4), it is possible to restrain an astigmatism, a field curvature, and a chromatic aberration within preferred ranges in a well-balanced manner, while restraining Petzval sum near zero. When the value exceeds the upper limit “−0.4”, the first lens has strong refractive power in relative to the second lens, an axial and an off-axis chromatic aberrations are insufficiently corrected. As for the astigmatism, a tangential image surface tilts to the object side and an astigmatic difference increases. For this reason, it is difficult to obtain satisfactory image-forming performance. On the other hand, when the value is below the lower limit “−1.0”, the first lens has weak refractive power in relative to the second lens, so that negative refractive power is strong and an axial and an off-axial chromatic aberrations are excessively corrected. In addition, an image-forming surface curves to the image plane side. As for the astigmatism, a tangential image surface tilts to an image plane side and the astigmatic difference increases. Therefore, also in this case, it is difficult to obtain satisfactory image-forming performance.

When a curvature radius of the object-side surface of the second lens is R 2 f and a curvature radius of the image plane-side surface thereof is R 2 r, the imaging lens having the above-described configuration preferably satisfies the following conditional expression (5):

1.5<R2f/R2r<6.0  (5)

When the imaging lens satisfies the conditional expression (5), it is possible to restrain a coma aberration, a field curvature, and a chromatic aberration within preferred ranges, respectively. When the value exceeds the upper limit “6.0”, the second lens has relatively strong refractive power, an outer coma aberration of an off-axis light beam increases, and an axial and an off-axis chromatic aberrations are excessively corrected. In addition, since an image-forming surface curves to the image plane side, it is difficult to obtain satisfactory image forming performance. On the other hand, when the value is below the lower limit “1.5”, the second lens has relatively weak refractive power, and the axial and the off-axis chromatic aberrations are insufficiently corrected. Moreover, the image forming surface curves to the object side, and also in this case, it is difficult to obtain satisfactory image forming performance.

When the whole lens system has the focal length f and the third lens has the focal length f 3 , the imaging lens having the above-described configuration preferably satisfies the following conditional expression (6):

5.0<f3/f<20.0  (6)

According to the imaging lens of the invention, the third lens mainly serves for correcting aberrations. When the imaging lens satisfies the conditional expression (6), it is possible to more satisfactorily correct aberrations while attaining miniaturization of the imaging lens. When the value exceeds the upper limit “20.0”, since the third lens has weak refractive power in relative to the whole lens system, it is difficult to attain miniaturization of the imaging lens. Here, when the third lens has weak refractive power, it is possible to attain miniaturization of the imaging lens by increasing the refractive power of the fourth lens or the fifth lens. In this case, however, it is difficult to correct aberrations (especially coma aberration), and it is difficult to obtain satisfactory image forming performance. On the other hand, when the value is below the lower limit “5.0”, although it is advantageous for attaining miniaturization of the imaging lens, the coma aberration increases and the astigmatic difference also increases. Therefore, also in this case, it is difficult to obtain satisfactory image forming performance.

When the maximum effective diameter of the object-side surface of the third lens is Φ 3A , the maximum effective diameter of the image plane-side surface thereof is Φ 3B , the maximum effective diameter of the object-side surface of the fourth lens is Φ 4A , the maximum effective diameter of the image plane-side surface thereof is Φ 4B , and the maximum absolute value of a sag (sagittal height) at up to 70% of the maximum effective diameters Φ 3A to Φ 4B is Z 0.7 , the imaging lens having the above-described configuration preferably satisfies the following conditional expression (7):

Z 0.7 /f<0.1  (7)

Therefore, restraining the maximum sag within certain range, the third lens and the fourth lens are formed in shapes that have substantially uniform thickness in the optical axis direction and are less curved. With such shapes of the third and the fourth lenses, generation of complicated aberrations is restrained and aberrations occurred in the first and the second lenses are satisfactorily corrected. Furthermore, sensitivity to deterioration of the image-forming performance due to decentering (axial displacement), tilting, or the like upon manufacturing the imaging lens, i.e., so-called “production error sensitivity” decreases. In addition, because of the substantially uniform thickness in the optical axis direction, fabrication properties upon production are improved and manufacturing cost of the imaging lens is restrained. Here, “sag” means, in each surface, a distance in a direction parallel to the optical axis from a tangential plane that is orthogonal to the optical axis to the surface.

When the whole lens system has the focal length f and a composite focal length of the third lens and the fourth lens is f 34 , the imaging lens having the above-described configuration preferably satisfies the following conditional expression (8):

5.0<f34/f<25.0  (8)

Restraining a ratio of the composite focal length of the third lens and the fourth lens to the focal length of the whole lens system within the range defined by the conditional expression (8), it is possible to more satisfactorily correct aberrations. When the value exceeds the upper limit “25.0”, the composite refractive power of the third lens and the fourth lens is relatively weak and it is difficult to restrain aberrations within preferred ranges in a well-balanced manner. On the other hand, when the value is below the lower limit “5.0”, composite refractive power of the third lens and the fourth lens is relatively strong, so although it is advantageous for correction of a distortion, the astigmatic difference increases. Therefore it is difficult to obtain satisfactory image-forming performance.

When a distance on the optical axis from the image plane-side surface of the second lens to the object-side surface of the third lens is dA and a distance on the optical axis from the image plane-side surface of the third lens to the object-side surface of the fourth lens is dB, the imaging lens having the above-described configuration preferably satisfies the following conditional expression (9):

0.3<dA/dB<1.5  (9)

In case of an imaging element such as a CCD sensor and a CMOS sensor, there is a limited range in an angle to take an incident light beam due to its structure. When an emitting angle of an off-axis light beam is outside the limited range above, it is difficult for a sensor to take therein light beams outside the limited range, which results in a so-called shading phenomenon. More specifically, an image obtained through the imaging lens has a dark periphery that is darker than the center part.

When the imaging lens satisfies the conditional expression (9), it is possible to keep the maximum emitting angle of an off-axis light beam small, while attaining miniaturization of the imaging lens. When the value exceeds the upper limit “1.5”, although it is easy to keep the maximum emitting angle of the off-axis light beam small, it is difficult to attain miniaturization of the imaging lens. On the other hand, when the value is below the lower limit “0.3”, although it is advantageous for miniaturization of the imaging lens, a chromatic aberration is insufficiently corrected and it is difficult to obtain satisfactory image-forming performance. In addition, since the maximum emitting angle of an off-axis light beam is large, the shading phenomenon easily occurs.

According to the imaging lens of the invention, it is possible to attain both miniaturization and satisfactory aberration correction of the imaging lens and provide a small-sized imaging lens with satisfactorily corrected aberrations.

FIG. 1 is a sectional view showing a schematic configuration of an imaging lens of Numerical Data Example 1 according to an embodiment of the invention;

FIG. 2 is an aberration diagram showing a lateral aberration of the imaging lens of FIG. 1 ;

FIG. 3 is an aberration diagram showing a spherical aberration, an astigmatism, and a distortion of the imaging lens of FIG. 1 ;

FIG. 4 is a sectional view showing a schematic configuration of an imaging lens of Numerical Data Example 2 according to the embodiment of the invention;

FIG. 5 is an aberration diagram showing a lateral aberration of the imaging lens of FIG. 4 ;

FIG. 6 is an aberration diagram showing a spherical aberration, an astigmatism, and a distortion of the imaging lens of FIG. 4 ;

FIG. 7 is a sectional view showing a schematic configuration of an imaging lens of Numerical Data Example 3 according to the embodiment of the invention;

FIG. 8 is an aberration diagram showing a lateral aberration of the imaging lens of FIG. 7 ;

FIG. 9 is an aberration diagram showing a spherical aberration, an astigmatism, and a distortion of the imaging lens of FIG. 7 ;

FIG. 10 is a sectional view showing a schematic configuration of an imaging lens of Numerical Data Example 4 according to an embodiment of the invention;

FIG. 11 is an aberration diagram showing a lateral aberration of the imaging lens of FIG. 10 ;

FIG. 12 is an aberration diagram showing a spherical aberration, an astigmatism, and a distortion of the imaging lens of FIG. 10 ;

FIG. 13 is a sectional view showing a schematic configuration of an imaging lens of Numerical Data Example 5 according to the embodiment of the invention;

FIG. 14 is an aberration diagram showing a lateral aberration of the imaging lens of FIG. 13 ; and

FIG. 15 is an aberration diagram showing a spherical aberration, an astigmatism, and a distortion of the imaging lens of FIG. 13 .

Hereunder, referring to the accompanying drawings, an embodiment of the present invention will be fully described.

FIGS. 1, 4, 7, 10, and 13 are schematic sectional views of imaging lenses in Numerical Data Examples 1 to 5 according to the embodiment, respectively. Since a basic lens configuration is the same among those Numerical Data Examples, the lens configuration of the embodiment will be described with reference to the schematic sectional view of Numerical Data Example 1.

As shown in FIG. 1 , the imaging lens of the embodiment includes a first lens L 1 having positive refractive power, a second lens L 2 having negative refractive power, a third lens L 3 having positive refractive power, a fourth lens L 4 having negative refractive power, and a fifth lens L 5 having negative refractive power, arranged in the order from an object side to an image plane side. A filter 10 such as an infrared cut-off filter or a cover glass may be provided between the fifth lens L 5 and an image plane IM. The filter 10 may be optionally omitted. Here, according to the imaging lens of this embodiment, there is provided an aperture stop on an object-side surface of the first lens L 1 .

According to the imaging lens of the embodiment, Abbe's numbers νd 1 and νd 3 to νd 5 of the first lens and the lenses from the third lens L 3 to the fifth lens L 5 are respectively larger than 45, and an Abbe's number νd 2 of the second lens L 2 is smaller than 35. More specifically, the first lens L 1 and the second lens L 2 are made from a combination of low-dispersion material and a high-dispersion material, and each of the lenses from the third lens L 3 to the fifth lens L 5 is made of a low-dispersion material. With this arrangement of the Abbe's numbers, a chromatic aberration occurred in the first lens L 1 is corrected by the second lens L 2 and also satisfactorily corrected through each of the lenses from the third lens L 3 to the fifth lens L 5 . In addition, since four of the five lenses from the first lens L 1 to the fifth lens L 5 are made of low-dispersion materials, generation of a chromatic aberration itself is suitably restrained. Here, more specifically, the Abbe's numbers νd 1 to νd 5 of the lenses from the first lens L 1 to the fifth lens L 5 are preferably restrained within ranges defined by the following conditional expressions, respectively. The imaging lenses of Numerical Data Examples 1 to 5 satisfy the following conditional expressions:

45<νd1<85

νd2<35

45<νd3<85

45<νd4<85

45<νd5<85

According to the imaging lenses in Numerical Data Examples 1 to 5 of this embodiment, the first lens L 1 and each of the lenses from the third lens L 3 to the fifth lens L 5 have the same Abbe's number, and each lens is made of a same plastic material. For this reason, it is possible to suitably attain productivity improvement and manufacturing cost reduction of the imaging lens.

Furthermore, each of the lenses from the first lens L 1 to the fifth lens L 5 satisfies the following conditional expressions (1) and (2):

f1<f3 and |f2|<f3  (1)

f3<|f4| and |f2|<|f5|  (2)

In the above conditional expressions:

f 1 : Focal length of a first lens L 1 f 2 : Focal length of a second lens L 2 f 3 : Focal length of a third lens L 3 f 4 : Focal length of a fourth lens L 4 f 5 : Focal length of a fifth lens L 5

As described above, according to the imaging lens of this embodiment, the first lens L 1 and the second lens L 2 , which are arranged on the object side in the imaging lens, have refractive powers that are stronger than those of other three lenses. Because of this, it is possible to attain miniaturization of the imaging lens and also suitably correct aberrations occurred in the first lens L 1 and the second lens L 2 through the respective lenses from the third lens L 3 to the fifth lens L 5 , which have weak refractive powers.

According to the imaging lens having the above-described configuration, the first lens L 1 is formed in a shape so that a curvature radius of an object-side surface thereof R 1 and a curvature radius of an image plane-side surface thereof R 2 are both positive, i.e. a shape of a meniscus lens directing a convex surface thereof to the object side near an optical axis X. Here, the shape of the first lens L 1 is not limited to a shape of the meniscus lens directing a convex surface thereof to the object side near the optical axis X, and can be any as long as the curvature radius of the object-side surface thereof R 1 is positive. More specifically, as a shape of the first lens L 1 , it is possible to form in a shape so that the curvature radius R 1 is positive and the curvature radius R 2 is negative, i.e. a shape of a biconvex lens near the optical axis X.

The second lens L 2 is formed in a shape of a meniscus lens so that a curvature radius of an object-side surface thereof R 3 and a curvature radius of an image plane-side surface thereof R 4 are both positive and directs a convex surface thereof to the object side near the optical axis X. In addition, as described above, the second lens L 2 is formed to satisfy the following conditional expressions (3) and (5). The second lens L 2 and the first lens L 1 are lenses having strong refractive powers among the lenses in the lens system. According to the embodiment, the first lens L 1 and the second lens L 2 satisfy the following conditional expression (4). When the imaging lens satisfies the conditional expression (4), it is possible to restrain Petzval sum of the whole lens system near zero and restrain an astigmatism, a field curvature, and a chromatic aberration within preferred ranges in a well-balanced manner.

−1.8<f2/f<−0.8  (3)

−1.0<f1/f2<−0.4  (4)

1.5<R2f/R2r<6.0  (5)

In the above conditional expressions:

R 2 f: Curvature radius of an object-side surface of the second lens R 2 r: Curvature radius of an image plane-side surface of the second lens

In order to more satisfactorily correct aberrations, the imaging lens preferably satisfies the following conditional expression (4A). The imaging lenses in Numerical Data Examples 1 to 5 satisfy the following conditional expression (4A):

−0.7<f1/f2<−0.4  (4A)

On the other hand, the third lens L 3 is formed in a shape so that a curvature radius of an object-side surface thereof R 5 is positive and a curvature radius of an image plane-side surface R 6 is negative, so as to have a shape of a biconvex lens near the optical axis X. Here, the shape of the third lens L 3 is not limited to a shape of a biconvex lens near the optical axis X and can be any as long as it is formed in a shape so that the curvature radius of the object surface thereof R 5 is positive. Numerical Data Examples 1 to 3 are examples in which the third lens L 3 has a shape of a biconvex lens near the optical axis X. Numerical Data Examples 4 and 5 are examples in which the third lens L 3 has a shape of a meniscus lens directing a convex surface thereof to the object side near the optical axis X.

The third lens L 3 satisfies the following conditional expression (6). With this configuration, it is possible to more satisfactorily correct aberrations. The imaging lenses in Numerical Data Examples 1 to 5 satisfy the following conditional expression (6):

5.0<f3/f<20.0  (6)

The fourth lens L 4 is formed in a shape so that a curvature radius of an object-side surface thereof R 7 and a curvature radius of an image plane-side surface thereof R 8 are both negative, i.e. a shape of a meniscus lens directing a concave surface thereof to the object side near the optical axis X. As described above, the second lens L 2 is formed in a shape of a meniscus lens directing a convex surface thereof to the object side near the optical axis X. Forming the fourth lens L 4 in a shape of a meniscus lens directing a concave surface thereof to the object side near the optical axis X, the second lens L 2 and the fourth lens L 4 are arranged directing their concave surfaces to the third lens L 3 . Therefore, aberrations occurred in the first lens L 1 are suitably corrected by the negative-positive-negative refractive power arrangement of the lenses from the second lens L 2 to the fourth lens L 4 and the shapes of the respective lens surfaces of the second lens L 2 and the fourth lens L 4 .

The third lens L 3 and the fourth lens L 4 mainly serve for correcting aberrations. According to the imaging lens of this embodiment, restraining the sag (sagittal height) of each lens surface of the third lens L 3 and the fourth lens L 4 within certain range, aberrations are satisfactorily corrected. More specifically, when the maximum effective diameter of the object-side surface of the third lens L 3 is Φ 3A , the maximum effective diameter of the image plane-side surface thereof is Φ 3B , the maximum effective diameter of the object-side surface of the fourth lens L 4 is Φ 4A , the maximum effective diameter of the image plane-side surface thereof is Φ 4B , and the maximum absolute value of a sag at up to 70% of the maximum effective diameters Φ 3A to Φ 4B is Z 0.7 , the imaging lens having the above-described configuration preferably satisfies the following conditional expression (7). The imaging lenses of Numerical Data Examples 1 to 5 satisfy the conditional expression (7):

Z 0.7 /f<0.1  (7)

With the maximum sags being restrained below the upper limit of the conditional expression (7), the third lens L 3 and the fourth lens L 4 have substantially uniform thicknesses in a direction of the optical axis X, and thereby have less curved shapes. With such lens shapes, aberrations are more satisfactorily corrected. Furthermore, sensitivity to deterioration of the image-forming performance due to decentering (axial displacement), tilting, or the like upon manufacturing the imaging lens, i.e. so-called “production error sensitivity” decreases. In addition, because of the substantially uniform thickness in the optical axis direction, fabrication properties upon production are improved and manufacturing cost of the imaging lens can be restrained. Here, “sag” means, in each surface, a distance in a direction parallel to the optical axis X from a tangential plane that is orthogonal to the optical axis X to the surface.

Here, each of lenses from the second lens L 2 to the fourth lens L 4 satisfies the following conditional expressions (8) and (9). With this configuration, aberrations are more satisfactorily corrected. In addition, since the maximum emitting angle of an off-axis light beam is kept small, generation of the shading phenomenon is restrained.

5.0<f34/f<25.0  (8)

0.3<dA/dB<1.5  (9)

In the above conditional expressions:

f 34 : Composite focal length of the third lens L 3 and the fourth lens L 4 dA: Distance on an optical axis from an image plane-side surface of the second lens L 2 to an object-side surface of the third lens L 3 dB: Distance on an optical axis from an image plane-side surface of the third lens L 3 to an object-side surface of the fourth lens L 4

The fifth lens L 5 is formed in a shape so that a curvature radius of an object-side surface thereof R 9 and a curvature radius of an image plane-side surface thereof R 10 are both positive, so as to have a shape of a meniscus lens directing a convex surface thereof to the object side near the optical axis X. In addition, an image plane-side surface of the fifth lens L 5 is formed as an aspheric shape so as to be convex to the object side near the optical axis X and concave to the object side at the periphery. With such shape of the fifth lens L 5 , it is possible to suitably restrain an incident angle of a light beam emitted from the imaging lens to the image plane IM.

Here, it is not necessary to satisfy all of the conditional expressions (1) to (9) and (4A), and it is possible to obtain an effect corresponding to the respective conditional expression when any single one of the conditional expressions is individually satisfied.

In the embodiment, each lens has lens surfaces that are formed to be an aspheric surface. When the aspheric surfaces applied to the lens surfaces have an axis Z in the optical axis direction, a height H in a direction perpendicular to the optical axis, a conical coefficient k, and aspheric coefficients A 4 , A 6 , A 8 , A 10 , A 12 , A 14 , and A 16 , a shape of the aspheric surfaces of the lens surfaces may be expressed as follows:

Next, Numerical Data Examples of the imaging lens of the embodiment will be described. In each Numerical Data Example, f represents a focal length of the whole lens system, Fno represents an F number, and ω represents a half angle of view, respectively. In addition, i represents a surface number counted from the object side, R represents a curvature radius, d represents a distance between lens surfaces (surface spacing) on the optical axis, Nd represents a refractive index for a d line (a reference wavelength in this embodiment), and νd represents the Abbe's number for the d line, respectively. Here, aspheric surfaces are indicated with surface numbers i affixed with * (asterisk).

NUMERICAL DATA EXAMPLE 1

The values of the respective conditional expressions are as follows:

f2/f=−1.59

f1/f2=−0.51

R2f/R2r=2.34

f3/f=6.39

Z 0.7 /f=0.017

f34/f=7.02

dA/dB=0.82

Accordingly, the imaging lens of Numerical Data Example 1 satisfies the above-described conditional expressions. A distance on the optical axis from an object-side surface of the first lens L 1 to an image plane IM (the thickness of the filter 10 is a length in air, which is hereinafter the same) is 4.41 mm, and miniaturization of the imaging lens is suitably attained.

FIG. 2 shows the lateral aberration that corresponds to a ratio H of each image height to the maximum image height (hereinafter referred to as “image height ratio H”) in the imaging lens of Numerical Data Example 1, which is divided into a tangential direction and sagittal direction (which is also the same in FIGS. 5, 8, 11, and 14 ). Furthermore, FIG. 3 shows a spherical aberration (mm), an astigmatism (mm), and a distortion (%) of the imaging lens of Numerical Data Example 1, respectively. In the aberration diagrams, for the lateral aberration diagrams and spherical aberration diagrams, aberrations at each wavelength, i.e. a g line (435.84 nm), an F line (486.13 nm), an e line (546.07 nm), a d line (587.56 nm), and a C line (656.27 nm), are indicated. In the astigmatism diagram, the aberrations on a sagittal image surface S and on a tangential image surface T are respectively indicated (which are the same in FIGS. 6, 9, 12, and 15 ). As shown in FIGS. 2 and 3 , in the imaging lens of Numerical Data Example 1, the aberrations are satisfactorily corrected.

NUMERICAL DATA EXAMPLE 2

The values of the respective conditional expressions are as follows:

f2/f=−1.57

f1/f2=−0.52

R2f/R2r=2.36

f3/f=6.52

Z 0.7 /f=0.017

f34/f=7.20

dA/dB=0.83

Accordingly, the imaging lens of Numerical Data Example 2 satisfies the above-described conditional expressions. A distance on the optical axis from an object-side surface of the first lens L 1 to the image plane IM is 4.42 mm, and miniaturization of the imaging lens is suitably attained.

FIG. 5 shows the lateral aberration that corresponds to the image height ratio H, and FIG. 6 shows a spherical aberration (mm), an astigmatism (mm), and a distortion (%), respectively, in the imaging lens of Numerical Data Example 2. As shown in FIGS. 5 and 6 , also in the imaging lens of Numerical Data Example 2, the image surface is satisfactorily corrected and the aberrations are suitably corrected similarly to Numerical Data Example 1.

NUMERICAL DATA EXAMPLE 3

The values of the respective conditional expressions are as follows:

f2/f=−1.44

f1/f2=−0.55

R2f/R2r=2.60

f3/f=5.58

Z 0.7 /f=0.013

f34/f=6.09

dA/dB=0.88

Accordingly, the imaging lens of Numerical Data Example 3 satisfies the above-described conditional expressions. A distance on the optical axis from an object-side surface of the first lens L 1 to the image plane IM is 4.35 mm, and miniaturization of the imaging lens is suitably attained.

FIG. 8 shows the lateral aberration that corresponds to the image height ratio H, and FIG. 9 shows a spherical aberration (mm), an astigmatism (mm), and a distortion (%), respectively, in the imaging lens of Numerical Data Example 3. As shown in FIGS. 8 and 9 , also in the imaging lens of Numerical Data Example 3, the image surface is satisfactorily corrected and the aberrations are suitably corrected similarly to Numerical Data Example 1.

NUMERICAL DATA EXAMPLE 4

The values of the respective conditional expressions are as follows:

f2/f=−1.80

f1/f2=−0.46

R2f/R2r=2.05

f3/f=16.23

Z 0.7 /f=0.010

f34/f=20.42

dA/dB=0.83

Accordingly, the imaging lens of Numerical Data Example 4 satisfies the above-described conditional expressions. A distance on the optical axis from an object-side surface of the first lens L 1 to the image plane IM is 4.45 mm, and miniaturization of the imaging lens is suitably attained.

FIG. 11 shows the lateral aberration that corresponds to the image height ratio H, and FIG. 12 shows a spherical aberration (mm), an astigmatism (mm), and a distortion (%), respectively, in the imaging lens of Numerical Data Example 4. As shown in FIGS. 11 and 12 , also in the imaging lens of Numerical Data Example 4, the image surface is satisfactorily corrected and the aberrations are suitably corrected similarly to Numerical Data Example 1.

NUMERICAL DATA EXAMPLE 5

The values of the respective conditional expressions are as follows:

f2/f=−1.17

f1/f2=−0.61

R2f/R2r=5.41

f3/f=7.59

Z 0.7 /f=0.016

f34/f=8.32

dA/dB=0.86

Accordingly, the imaging lens of Numerical Data Example 5 satisfies the above-described conditional expressions. A distance on the optical axis from an object-side surface of the first lens L 1 to the image plane IM is 4.49 mm, and miniaturization of the imaging lens is suitably attained.

FIG. 14 shows the lateral aberration that corresponds to the image height ratio H, and FIG. 15 shows a spherical aberration (mm), an astigmatism (mm), and a distortion (%), respectively, in the imaging lens of Numerical Data Example 5. As shown in FIGS. 14 and 15 , also in the imaging lens of Numerical Data Example 5, the image surface is satisfactorily corrected and the aberrations are suitably corrected similarly to Numerical Data Example 1.

Accordingly, when the imaging lens of the embodiment is applied to an imaging optical system of a cellular phone, a digital still camera, a portable information terminal, a security camera, a vehicle onboard camera, a network camera, and the like, it is possible to achieve both the high performance and the small size for the camera or the like.

Here, the imaging lens of the invention is not limited to the above-described embodiment. In the above-described embodiment, any surfaces of the first lens L 1 through the fifth lens L 5 are formed as aspheric surfaces, but it is not necessary to form all the surfaces as aspheric surfaces. Alternatively, it is also possible to form one of or both surfaces of any lens from the first lens L 1 through the fifth lens in a spherical surface(s).

The invention may be applicable to the imaging lens of a device that is required to have a small size and satisfactory aberration correction ability, e.g., the imaging lenses used in the cellular phones, the digital still cameras, and the like.

The disclosure of Japanese Patent Application No. 2011-190560, filed on Sep. 1, 2011, is incorporated in the application.

While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.