Imaging lens and imaging apparatus

The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2017-176474, filed on Sep. 14, 2017. The above application 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, particularly, relates to an imaging lens suitable for digital cameras, video cameras, and/or the like, and an imaging apparatus comprising the imaging lens.

2. Description of the Related Art

In recent years, an inner focusing system is used in an imaging lens used in an imaging apparatus such as a digital camera. For example, JP2012-159613A, JP2012-226309A, JP2012-242472A, JP2013-37080A, and JP2012-242689A each disclose an imaging lens that has a three-group configuration consisting of a first lens group, a second lens group, and a third lens group and performs focusing by moving the second lens group with respect to the image plane in a state where the first lens group and the third lens group remain stationary with respect to the image plane.

In the inner focus type imaging lens, in order to increase the autofocus speed and reduce a load on the focus driving system, it is desirable that a lens group (hereinafter referred to as a focus lens group) moving during focusing is configured to be lighter in weight. In the imaging lenses described in JP2012-159613A, JP2012-226309A, JP2012-242472A, JP2013-37080A, and JP2012-242689A, the focus lens group is composed of only one lens.

It is necessary to form the focus lens group with a minimum number of lenses. Further, in order to suppress fluctuation in aberrations caused by movement of the focus lens group, it is necessary to optimize the refractive power of the focus lens group and the configuration of the lens groups disposed on the object side and the image side thereof.

In the imaging lens according to JP2012-159613A, JP2012-226309A, JP2012-242472A, and JP2013-37080A, the third lens group, which is disposed to be closer to the image side than the focus lens group, is composed of only one positive lens. In such a configuration, it is difficult to suppress fluctuation in off-axis rays during focusing. Further, in a case where the third lens group is composed of only the positive lens, it is difficult to suppress fluctuation in chromatic aberration during focusing.

In the imaging lens described in JP2012-242689A, the third lens group is formed of two lenses including a positive lens and a negative lens. However, a refractive power of the second lens group is set to be strong, and thus fluctuation in off-axis rays during focusing becomes large.

The present invention has been made in consideration of the above-mentioned situation, and its object is to provide an inner focus type imaging lens that is capable of high-speed focusing and capable of suppressing fluctuation in aberrations during focusing so as to have favorable optical performance, and an imaging apparatus comprising the imaging lens.

An imaging lens of the present invention consists of, in order from an object side: a first lens group that has a positive refractive power; a second lens group that has a negative refractive power; and a third lens group that has a refractive power. The first lens group consists of, in order from the object side, a front group having a positive refractive power, a stop, and a rear group having a positive refractive power. The front group has at least one negative lens and at least one positive lens. The rear group has a cemented lens consisting of a negative lens concave toward an image side and a positive lens convex toward the image side in order from the object side. The second lens group consists of one negative lens. The third lens group has at least one negative lens and at least one positive lens. During focusing from an object at infinity to a close-range object, the first lens group and the third lens group remain stationary with respect to an image plane, and the second lens group moves from the object side to the image side. Assuming that a focal length of the second lens group is f2 and a focal length of the whole system in a state where the object at infinity is in focus is f, Conditional Expression (1) is satisfied.

0.8<| f 2/ f|< 5  (1)

It is preferable that the imaging lens of the present invention satisfies Conditional Expression (1-1).

1<| f 2/ f|< 4  (1-1)

In the imaging lens of the present invention, assuming that a focal length of the first lens group is f1, it is preferable to satisfy Conditional Expression (2), and it is more preferable to satisfy Conditional Expression (2-1).

0.4< f 1/ f< 1  (2)

0.5< f 1/ f< 0.9  (2-1)

In the imaging lens of the present invention, assuming that a refractive index of the negative lens of the second lens group at a d line is NdG2 and an Abbe number of the negative lens of the second lens group at the d line is νdG2, it is preferable to satisfy Conditional Expressions (3) and (4). Further, it is more preferable to satisfy at least one of Conditional Expressions (3-1) and (4-1) after satisfying Conditional Expressions (3) and (4).

1.55< NdG 2  (3)

30< νdG 2<65  (4)

1.55< NdG 2<2  (3-1)

32< νdG 2<62  (4-1)

In the imaging lens of the present invention, assuming that a focal length of the third lens group is f3, it is preferable to satisfy Conditional Expression (5), and it is more preferable to satisfy Conditional Expression (5-1).

| f 2/ f 3|<0.67  (5)

| f 2/ f 3|<0.6  (5-1)

In the imaging lens of the present invention, it is preferable that a negative lens is disposed to be closest to the object side in the front group. Further, it is preferable that the front group consists of a negative lens, which is disposed to be closest to the object side, and a front group sub-lens group which is disposed to be separated by an air gap from the negative lens and has a positive refractive power.

In the imaging lens of the present invention, it is preferable that the rear group has at least one aspheric surface.

In the imaging lens of the present invention, it is preferable that the third lens group consists of one negative lens and one positive lens. In this case, it is preferable that the third lens group consists of, in order from the object side, one negative lens and one positive lens. Further, in the imaging lens of the present invention, the third lens group may be configured to have a positive refractive power.

An imaging apparatus of the present invention comprises the imaging lens of the present invention.

In the present description, it should be noted that the terms “consisting of ˜” and “consists of ˜” are used in a substantial sense, and mean that the imaging lens may include not only the above-mentioned elements but also lenses substantially having no refractive powers, optical elements, which are not lenses, such as a stop, a filter, and a cover glass, and mechanism parts such as a lens flange, a lens barrel, an imaging element, and a camera shaking correction mechanism.

In the present specification, it should be noted that the “˜ group having a positive refractive power” means that the group has a positive refractive power as a whole. Likewise, the “˜ group having a negative refractive power” means that the group has a negative refractive power as a whole. Signs of refractive powers and surface shapes of the lenses are assumed as those in paraxial regions in a case where some lenses have aspheric surfaces. The “lens group” is not necessarily composed of a plurality of lenses, but may be composed of only one lens. The number of lenses mentioned above is the number of lenses formed as the elements. For example, the number of lenses of the cemented lens, in which a plurality of single lenses having different materials is cemented, indicates the number of single lenses constituting the cemented lens. However, a compound aspheric lens (a lens which is integrally composed of a spherical lens and a film having an aspheric shape formed on the spherical lens, and functions as one aspheric lens as a whole) is not be considered as a cemented lens, and is treated as a single lens. All the above-mentioned conditional expressions are based on the d line (a wavelength of 587.6 nm (nanometers)).

According to the present invention, the imaging lens consists of, in order from the object side, a first lens group having a positive power, a second lens group having a negative power, and a third lens group. In a lens system in which only the second lens group moves during focusing, a configuration of each lens group is appropriately set such that predetermined conditional expressions are further satisfied. Therefore, it is possible to provide an inner focus type imaging lens that is capable of high-speed focusing and capable of suppressing fluctuation in aberrations during focusing so as to have favorable optical performance, and an imaging apparatus comprising the imaging lens.

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

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

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

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

FIG. 5 is a cross-sectional view illustrating optical paths of the imaging lens shown in FIG. 1 .

FIG. 6 is a diagram of aberrations of the imaging lens of Example 1 of the present invention.

FIG. 7 is a diagram of aberrations of the imaging lens of Example 2 of the present invention.

FIG. 8 is a diagram of aberrations of the imaging lens of Example 3 of the present invention.

FIG. 9 is a diagram of aberrations of the imaging lens of Example 4 of the present invention.

FIG. 10 is a perspective view of the front side of an imaging apparatus according to an embodiment of the present invention.

FIG. 11 is a perspective view of the rear side of an imaging apparatus according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described with reference to drawings. FIGS. 1 to 4 are cross-sectional views illustrating configurations of imaging lenses according to an embodiment of the present invention, and respectively correspond to Examples 1 to 4 to be described later. FIGS. 1 to 4 show situations where an object at infinity is in focus, the left side thereof is an object side, and the right side thereof is an image side. Further, FIG. 5 is an optical path diagram of the imaging lens shown in FIG. 1 , and shows a lens configuration thereof and on-axis rays 2 and rays with the maximum angle of view 3. In FIG. 5 , a state where an object at infinity is in focus is shown in the upper part labeled as “infinity”, and a state where a close-range object having an object distance of “0.4 m” is in focus is shown in the lower part labeled as “0.4 m (meters)”. Basic configurations and illustration methods of examples shown in FIGS. 1 to 4 are the same, and will be hereinafter described with reference to mainly the example shown in FIG. 1 .

This imaging lens consists of, in order from the object side to the image side along an optical axis Z: a first lens group G 1 that has a positive refractive power as a whole; a second lens group G 2 that has a negative refractive power as a whole; and a third lens group G 3 that has a refractive power. In the example shown in FIG. 1 , the first lens group G 1 consists of, in order from the object side, six lenses, that is, lenses L 11 to L 16 , the second lens group G 2 consists of only one lens, that is, a lens L 21 , and the third lens group G 3 consists of, in order from the object side, two lenses, that is, lenses L 31 and L 32 .

FIG. 1 shows an example in which an optical member PP having a plane parallel plate shape is disposed between the lens system and the image plane Sim. The optical member PP is assumed to be an infrared cut filter, a low pass filter, various other filters, a cover glass, and/or the like. In the present invention, the optical member PP may be disposed at a position different from that in the example of FIG. 1 , or the optical member PP may be omitted.

The imaging lens is configured such that, during focusing from an object at infinity to a close-range object, the first lens group G 1 and the third lens group G 3 remain stationary with respect to the image plane Sim, and the second lens group G 2 is moved from the object side to the image side. Further, the second lens group G 2 is configured to consist of only one negative lens. The arrow under the second lens group G 2 of FIG. 1 indicates a direction in which the second lens group G 2 move during focusing from the object at infinity to the close-range object.

By employing an inner focusing system, it is possible to reduce a load to a system in which a focus lens unit is driven, and it is possible to reduce a size of the focus lens unit. As a result, it is possible to reduce a size of the whole lens system. In order to cope with the demand for reduction in size of the imaging apparatus, there is also a demand for reduction in size of the lens system. Further, by adopting a configuration in which the focus lens group is composed of one lens, it is possible to achieve reduction in weight thereof, and it is possible to achieve high-speed focusing. Furthermore, by making the reference signs of a refractive power of the second lens group G 2 as the focus lens group and a refractive power of the first lens group G 1 different from each other, it is possible to make the refractive power of the focus lens group strong, and it is possible to decrease an amount of movement of the focus lens group during focusing.

The third lens group G 3 is configured to have at least one negative lens and at least one positive lens. By arranging the positive lens and the negative lens in the third lens group G 3 , there is an advantage in correcting chromatic aberration and field curvature.

The imaging lens is configured such that the focal length of the second lens group G 2 satisfies Conditional Expression (1).

0.8<| f 2/ f|< 5  (1)

Here, f2 is a focal length of the second lens group, and f is a focal length of the whole system in a state where the object at infinity is in focus.

By not allowing the result of the conditional expression (1) to be equal to or less than the lower limit, the refractive power of the second lens group G 2 is prevented from becoming excessively strong. Thus, it is possible to satisfactorily perform aberration correction even in a case where the second lens group G 2 is composed of one lens. In particular, this is advantageous in suppressing occurrence of field curvature. Further, it is possible to increase the tolerance of the eccentricity error of the focus lens group which is a movable part. As a result, it becomes easier to realize favorable performance. By not allowing the result of the conditional expression (1) to be equal to or greater than the upper limit, it is possible to minimize the amount of movement of the second lens group G 2 during focusing. As a result, there is an advantage in reducing the size of the whole lens system and increasing the speed of focusing. In order to enhance the effect relating to Conditional Expression (1), it is more preferable to satisfy Conditional Expression (1-1).

1<| f 2/ f|< 4  (1-1)

The first lens group G 1 consists of, in order from the object side, a front group G 1 F that has a positive refractive power as a whole, an aperture stop St, and a rear group G 1 R that has a positive refractive power as a whole. In the example shown in FIG. 1 , the front group G 1 F consists of lenses L 11 to L 13 , and the rear group G 1 R consists of lenses L 14 to L 16 . It should be noted that the aperture stop St shown in FIG. 1 does not necessarily indicate its sizes and/or shapes, and indicates a position of the aperture stop St on the optical axis Z.

By forming the front group G 1 F, which is a lens group closest to the object side, as a lens group having a positive refractive power, it is possible to shorten a total length of the lens system. The aperture stop St may be disposed between the lenses constituting the first lens group G 1 , and positions of the lens closest to the object side and the aperture stop St may be set to be close to each other, whereby it is possible to reduce a diameter of the lens closest to the object side.

The front group G 1 F is configured to have at least one negative lens and at least one positive lens. By arranging the positive lens and the negative lens in the front group G 1 F, there is an advantage in correcting chromatic aberration and field curvature. It is preferable that the negative lens is disposed to be closest to the object side in the front group G 1 F. By arranging the negative lens such that it is closest to the object side, it is possible to achieve the wide angle without increasing the diameter of the lens closest to the object side.

The front group G 1 F may be configured to consist of a front group Fa sub-lens group G 1 Fa, which consists of one negative lens disposed to be closest to the object side, and a front group Fb sub-lens group G 1 Fb which is disposed to be separated by an air gap from the negative lens and has a positive refractive power as a whole. With such a configuration, the front group G 1 F serves as a wide converter, and thus it is possible to achieve the wide angle while correcting distortion or coma aberration.

In a case where the front group G 1 F is configured to consist of the front group Fa sub-lens group G 1 Fa and the front group Fb sub-lens group G 1 Fb, the front group Fb sub-lens group G 1 Fb may be a cemented lens that is formed by cementing one positive lens and one negative lens. In such a case, by providing the cemented lens separately from the negative lens closest to the object side, it is possible to correct chromatic aberration.

The rear group G 1 R is configured to have a cemented lens consisting of a negative lens concave toward the image side and a positive lens convex toward the image side in order from the object side. By arranging the positive lens and the negative lens in the rear group G 1 R, there is an advantage in correcting chromatic aberration and field curvature. Further, since the rear group G 1 R has a cemented lens consisting of two lenses, it is possible to shorten the total length of the lens system while correcting chromatic aberration using the minimum number of lenses that is capable to composing the cemented lens.

The second lens group G 2 consists of one negative lens. It is preferable that the negative lens composing the second lens group G 2 is an aspheric lens including at least one aspheric surface. In this case, fluctuation in field curvature during focusing can be suppressed. Furthermore, in a case where both surfaces on the object side and the image side of the negative lens composing the second lens group G 2 are aspheric, it is possible to more satisfactorily suppress the fluctuation in field curvature.

The rear group G 1 R also preferably has at least one aspheric surface. In a case where the second lens group G 2 is composed of an aspheric lens and has a function of suppressing fluctuation in field curvature during focusing, field curvature, which remains after being corrected by the second lens group G 2 , can be corrected by the aspheric surface of the rear group G 1 R. Further, the rear group G 1 R has a cemented lens formed by cementing a negative lens concave toward the image side and a positive lens convex toward the image side in order from the object side, and includes an aspheric surface. Thereby, it is possible to satisfactorily correct chromatic aberration and field curvature while reducing the size of the lens system with a small number of lenses.

The third lens group G 3 may be a lens group having a positive refractive power, and may be a lens group having a negative refractive power. However, in a case where the third lens group G 3 is formed as a lens group having a positive refractive power, by separating an exit pupil position from the image plane Sim, it is possible to decrease an angle of incidence of rays which is incident on the image plane Sim. As a result, in a case where a solid-state imaging device is disposed at a position of the image plane Sim of the imaging lens, this configuration is advantageous in favorable image acquisition. Further, in a case where the third lens group G 3 is formed as a lens group having a positive refractive power, reference signs of the refractive power of the third lens group G 3 and the refractive power of the second lens group G 2 as the focus lens group are different. Thus, it is possible to increase the refractive power of the focus lens group, and it is possible to decrease an amount of movement of the focus lens group during focusing.

The third lens group G 3 may be configured to consist of one negative lens and one positive lens. By making the third lens group G 3 include the two lenses and arranging the positive lens and the negative lens, there is an advantage in correcting chromatic aberration and field curvature. At that time, the third lens group G 3 may be configured to consist of, in order from the object side, one negative lens and one positive lens. In such a case of using an arrangement order, it is possible to decrease the angle of incidence of rays which are incident on the image plane Sim. Further, in a case where the third lens group G 3 consists of one negative lens and one positive lens, these two lenses may be cemented to each other. In addition, in a case where the lenses are cemented, there is an advantage in correcting chromatic aberration.

Next, preferred configurations relating to conditional expressions will be described. It is preferable that the imaging lens satisfies at least one or an optional combination of Conditional Expressions (2) to (5).

0.4< f 1/ f< 1  (2)

1.55< NdG 2  (3)

30<ν dG 2<65  (4)

| f 2/ f 3|<0.67  (5)

Here, f1 is a focal length of the first lens group, f is a focal length of the whole system in a state where the object at infinity is in focus, NdG2 is a refractive index of the negative lens of the second lens group at the d line, νdG2 is an Abbe number of the negative lens of the second lens group at the d line, f2 is a focal length of the second lens group, and f3 is a focal length of the third lens group.

By not allowing the result of the conditional expression (2) to be equal to or less than the lower limit, the refractive power of the first lens group G 1 is prevented from becoming excessively strong, and thus there is an advantage in correcting spherical aberration and coma aberration occurring in the first lens group G 1 . By not allowing the result of the conditional expression (2) to be equal to or greater than the upper limit, the refractive power of the first lens group G 1 is prevented from becoming excessively weak, and thus there is an advantage in reducing the size of the optical system. In order to enhance the effect relating to Conditional Expression (2), it is more preferable that Conditional Expression (2-1) is satisfied.

0.5< f 1/ f< 0.9  (2-1)

By not allowing the result of the conditional expression (3) to be equal to or less than the lower limit, it is possible to increase a proportion of an amount of movement of an image plane position to the amount of movement of the focus lens group in the optical axis direction while minimizing aberrations.

By not allowing the result of the conditional expression (4) to be equal to or less than the lower limit, it is possible to suppress fluctuation in chromatic aberration caused by movement of the focus lens group. By not allowing the result of the conditional expression (4) to be equal to or greater than the upper limit, there is an advantage in correcting lateral chromatic aberration caused by the first lens group G 1 . In order to enhance the effect relating to Conditional Expression (4), it is more preferable that Conditional Expression (4-1) is satisfied.

32<ν dG 2<62  (4-1)

In the configuration of the focus lens group, it is preferable that Conditional Expressions (3) and (4) are satisfied, and it is more preferable that Conditional Expressions (3) and (4-1) are satisfied. In addition, in order to use an optical material which satisfies Conditional Expressions (3) and (4), it is preferable that Conditional Expression (3-1) is satisfied.

1.55< NdG 2<2  (3-1)

By not allowing the result of Conditional Expression (5) to be equal to or greater than the upper limit, the refractive power of the third lens group G 3 is prevented from becoming excessively strong. As a result, it is possible to minimize change in off-axis rays during focusing. Accordingly, it is possible to suppress fluctuation in field curvature during focusing. Further, by not allowing the result of Conditional Expression (5) to be equal to or greater than the upper limit, the refractive power of the second lens group G 2 as the focus lens group is prevented from becoming excessively weak, and thus there is an advantage in a decrease in amount of movement of the focus lens group. The lower limit of |f2/f3| is represented by 0≤|f2/f3|. In order to enhance the effect of Conditional Expression (5), it is preferable that Conditional Expression (5-1) is satisfied.

| f 2/ f 3|<0.6  (5-1)

The above-mentioned preferred configurations and available configurations may be optional combinations, and it is preferable to selectively adopt the configurations in accordance with required specification.

Next, numerical examples of the imaging lens of the present invention will be described.