阅读说明：本技术 透镜系统和摄像装置 (Lens system and image pickup apparatus ) 是由 矢崎卓也 于 2018-12-26 设计创作，主要内容包括：摄像用的透镜系统(10)具有从物体侧(11)起依次配置的在聚焦时移动的正折射力的第一透镜组(G1)、以与第一透镜组(G1)将光圈(St)夹在中间的方式配置且在聚焦时移动的正折射力的第二透镜组(G2)以及配置于最靠像面侧(12)的位置且被固定的正折射力的第三透镜组(G3)。第三透镜组包括由物体侧起依次配置的正折射力的透镜和负折射力的透镜构成的接合透镜(B31),第三透镜组的合成焦距(f3)与第一透镜组及第二透镜组的合成焦距(f12)满足下面的条件。2≤f3/f12≤200。(A lens system (10) for imaging is provided with a first lens group (G1) having positive refractive power and moving during focusing, a second lens group (G2) having positive refractive power and moving during focusing, the first lens group (G1) and the second lens group being arranged in this order from the object side (11), the second lens group being arranged so as to sandwich a diaphragm (St), and a third lens group (G3) having positive refractive power and being fixed at a position closest to the image surface side (12). The third lens group includes a cemented lens (B31) composed of a positive refractive power lens and a negative refractive power lens arranged in this order from the object side, and the combined focal length (f3) of the third lens group and the combined focal length (f12) of the first lens group and the second lens group satisfy the following conditions. F3/f12 is more than or equal to 2 and less than or equal to 200.)

1. A lens system for use in image pickup,

a first lens group having positive refractive power which moves during focusing, a second lens group having positive refractive power which is disposed in order from the object side so as to sandwich a diaphragm with the first lens group and moves during focusing, and a third lens group having positive refractive power which is disposed at the position closest to the image surface side and fixed,

the third lens group includes a cemented lens composed of a positive refractive power lens and a negative refractive power lens arranged in this order from the object side, and a combined focal length f3 of the third lens group and a combined focal length f12 of the first lens group and the second lens group satisfy the following conditions:

2≤f3/f12≤200。

2. the lens system of claim 1,

the refractive index nB31a of the positive refractive power lens among the cemented lenses satisfies the following condition:

1.65≤nB31a≤2.0。

3. the lens system of claim 1 or 2,

a refractive index nB31ab of at least one of the positive refractive power lens and the negative refractive power lens of the cemented lens, a distance G3L on the optical axis from the most object side surface to the most image surface side surface of the third lens group, and a distance B31L on the optical axis of the cemented lens satisfy the following conditions:

0·6≤B31L/G3L≤1

1.8≤nB31ab≤2.0。

4. the lens system of claim 3,

the refractive index nB31a of the positive refractive power lens among the cemented lenses and the distance B31L of the cemented lens satisfy the following condition:

0.65≤B31L/G3L≤0.80

1.8≤nB31a≤2.0。

5. the lens system of any of claims 1 to 4,

the refractive index nB31b of the negative refractive power lens among the cemented lenses satisfies the following condition:

1.60≤mB31b＜1.87。

6. the lens system of any of claims 1 to 5,

the refractive index nB31a of the positive refractive power lens among the cemented lenses satisfies the following condition:

1.85≤nB31a≤2.0。

7. the lens system of any of claims 1 to 6,

the third lens group includes the cemented lens arranged from the object side and a lens behind a negative refractive power recessed on the object side.

8. The lens system of claim 7,

a combined focal length f31ab of a positive refractive power lens and a negative refractive power lens of the cemented lenses and a focal length f3GL of the rear lens satisfy the following condition:

0.5≤|f31ab/f3GL|≤1.1。

9. the lens system according to claim 7 or 8,

a combined focal length f3ab of a positive refractive power lens and a negative refractive power lens of the cemented lenses, a focal length f3GL of the rear lens, and a combined focal length f3 of the third lens group satisfy the following conditions:

0＜(|f31ab|+|f3GL|)/|f3|≤1.3。

10. the lens system of any of claims 7 to 9,

a refractive index nB31b of a negative refractive power lens of the cemented lenses and a refractive index n3GL of the rear lens satisfy the following condition:

0.5＜n3GL/nB31b＜1。

11. the lens system of any of claims 1 to 10,

the first lens group includes a first cemented lens composed of a negative refractive power lens and a positive refractive power lens and a second cemented lens composed of a positive refractive power lens and a negative refractive power lens arranged in this order from the object side,

the second lens group includes a third cemented lens composed of a negative refractive power lens and a positive refractive power lens and a rear positive refractive power lens arranged in this order from the object side,

the third lens group includes the cemented lens as a fourth cemented lens,

a refractive index nB11b of a positive refractive power lens of the first cemented lens and a refractive index nB31a of a positive refractive power lens of the fourth cemented lens satisfy the following condition:

1.75≤nB11b≤2.0

1.75≤nB31a≤2.0。

12. the lens system of claim 11,

the refractive index nB31a of the positive refractive power lens in the fourth cemented lens, the refractive index nB31B of the negative refractive power lens in the fourth cemented lens, the abbe number ν B11a of the negative refractive power lens in the first cemented lens, and the abbe number ν B11B of the positive refractive power lens in the first cemented lens satisfy the following conditions:

0.5＜nB31b/nB31a＜1

0.5＜vB11a/vB11b＜1。

13. the lens system of claim 11 or 12,

the positive refractive power lens of the third cemented lens is composed of an abnormally low dispersion glass.

14. The lens system of any of claims 11 to 13,

the lens of positive refractive power at the rear of the third cemented lens is composed of an abnormally low dispersion glass.

15. The lens system of any of claims 1 to 14,

a combined focal length f3 of the third lens group and a combined focal length f12 of the first lens group and the second lens group satisfy the following condition:

100≤f3/f12≤170。

16. the lens system of any of claims 1 to 14,

a combined focal length f3 of the third lens group and a combined focal length f12 of the first lens group and the second lens group satisfy the following condition:

2≤f3/f12≤10。

17. the lens system of any of claims 1 to 16,

the second lens group includes at least one lens made of an abnormally low dispersion glass.

18. The lens system of any of claims 1 to 17,

the second lens group includes at least two lenses made of anomalous low dispersion glass.

19. The lens system of any of claims 1 to 18,

the first lens group, the diaphragm, and the second lens group move integrally upon focusing.

20. An image pickup apparatus includes:

a lens system according to any one of claims 1 to 19; and

and an imaging element disposed on an image plane side of the lens system.

Technical Field

The present invention relates to a lens system and an image pickup apparatus.

Background

Japanese patent laid-open publication No. 2014-126652 describes the following: the first lens group is composed of a first lens group having positive refractive power and including an aperture stop, and a second lens group having negative refractive power, the first lens group being moved to the object side in accordance with focusing from infinity to a close distance, the first lens group being composed of a first lens group front group having positive refractive power on the object side of the aperture stop and a first lens group rear group having positive refractive power on the image side of the aperture stop, the first lens group front group including a positive lens, and a cemented lens including a positive lens having a convex surface facing the object side and a negative lens having a concave surface facing the image side at a position on the image side of the positive lens.

Disclosure of Invention

In medium telephoto or standard type lenses, high performance lens systems are sought.

One aspect of the present invention is a lens system for imaging, including, arranged in order from an object side, a first lens group of positive refractive power that moves during focusing, a second lens group of positive refractive power that is arranged so as to sandwich a diaphragm with the first lens group and moves during focusing, and a third lens group of positive refractive power that is arranged at a position closest to an image surface side and is fixed. The third lens group includes a cemented lens composed of a positive refractive power lens and a negative refractive power lens arranged in this order from the object side, and a composite focal length f3 of the third lens group and a composite focal length f12 of the first lens group and the second lens group satisfy the following conditions.

2≤f3/f12≤200

The lens system is a positive-positive 3-group configuration. Of the types in which the object side is provided with positive power, a telephoto type (telephoto type) in which negative power is provided on the image plane side is a general case, and a compact lens system can be provided in a standard type to the telephoto type. On the other hand, in order to diffuse the light flux converged by the lens having positive power on the object side to the image plane by the negative power on the rear side, the amount of refraction of the light beam at each lens becomes large, and particularly the amount of refraction at the lens on the object side where the positive power is concentrated becomes large, and it is difficult to perform aberration correction. In order to perform the aberration correction satisfactorily, many surfaces are required, the number of lenses tends to increase, and if the number of lenses increases, the influence of individual differences or tolerances becomes large. Further, if the number of lens pieces is increased, the MTF is likely to be decreased, and thus, although the MTF is increased in design, if not many lenses are arranged at predetermined positions with predetermined accuracy, the MTF is likely to be decreased.

In the lens system of this example, the power arrangement of the telephoto type, positive-negative, is a 3-group configuration of positive powers, positive-positive, so that the positive powers are dispersed in 3 lens groups. This makes it possible to avoid concentration of positive power in any lens group, particularly in the lens group on the object side, and to suppress the occurrence of aberration, and to perform aberration correction with a small number of lenses. The third lens group closest to the image plane has a lower positive power than the other lens groups on the object side, thereby providing a configuration suitable for intermediate telephoto. Therefore, the combined focal length f3 of the third lens group and the combined focal length f12 of the first lens group and the second lens group satisfy the above condition.

In addition, a cemented lens including a combination of a cemented surface having a certain degree of curvature and a surface having a large curvature that can secure a distance from the cemented surface is used in the third lens group, and correction of each aberration including chromatic aberration is performed by the cemented lens without adjusting the surface interval. Therefore, in the third lens group, the ratio occupied by the cemented lens is very large. On the other hand, if the distance (length) of the cemented lens is excessively large, the total length of the lens system is excessively long, and the curvature of the cemented surface is also excessively large, so that the manufacturing cost increases. Therefore, the cemented lens of the third lens group is made of glass having a high refractive index, and a cemented lens having a predetermined aberration correction capability is compactly mounted.

Therefore, it may be that the distance on the optical axis (the total length of the third lens group) G3L of the third lens group and the distance on the optical axis (the length of the cemented lens) B31L of the cemented lens satisfy the following condition, and the refractive index nB31ab of at least one of the positive refractive power lens and the negative refractive power lens of the cemented lens satisfies the following condition.

0.6≤B31L/G3L≤1

1.8≤nB31ab≤2.0

The refractive index nB31a of the lens of positive refractive power among the cemented lenses may satisfy the following condition.

1.65≤nB31a≤2.0

In this lens system, a cemented lens including a cemented surface having a certain degree of curvature is used as the third lens group, and correction of each aberration including chromatic aberration is performed. Therefore, in the third lens group, the ratio occupied by the cemented lens is very large. On the other hand, if the distance (length) of the cemented lens is excessively large, the total length of the lens system is excessively long, and the curvature of the cemented surface is also excessively large, so that the manufacturing cost increases. Therefore, the positive refractive power lens having the refractive index nB31a under the above conditions is adopted as the cemented lens, and the cemented lens having a predetermined aberration correction capability is compactly mounted.

Another aspect of the present invention is a lens system for image pickup including a first lens group of positive refractive power, a second lens group of positive refractive power, and a third lens group of positive refractive power, which are arranged in this order from an object side, the second lens group of positive refractive power being arranged so as to sandwich a diaphragm with the first lens group. The first lens group includes a first cemented lens composed of a negative refractive power lens and a positive refractive power lens and a second cemented lens composed of a positive refractive power lens and a negative refractive power lens arranged in this order from the object side, the second lens group includes a third cemented lens composed of a negative refractive power lens and a positive refractive power lens and a rear positive refractive power lens arranged in this order from the object side, the third lens group includes a fourth cemented lens composed of a positive refractive power lens and a negative refractive power lens arranged in this order from the object side, and the refractive index nB11b of the positive refractive power lens in the first cemented lens and the refractive index nB31a of the positive refractive power lens in the fourth cemented lens satisfy the following conditions.

1.75≤nB11b≤2.0

1.75≤nB31a≤2.0

The lens system of this example has a 3-group configuration of positive powers such as positive-positive power, in which the power arrangement of positive-negative power of the telephoto type is set, and the positive powers are dispersed in 3 lens groups. This makes it possible to avoid concentration of positive power in any lens group, particularly in the lens group on the object side, and to suppress the occurrence of aberration, and to perform aberration correction with a small number of lenses.

Further, by disposing the first cemented lens composed of a combination of a negative lens and a positive lens at the most object side position of the first lens group and disposing the fourth cemented lens having a combination of symmetrical powers at a position symmetrical to the first cemented lens, the symmetry of the lens system can be improved, and this is also effective for reduction of petzval sum. Of these first and fourth cemented lenses, lenses having positive power with a large distance (thickness) along the optical axis are desired to have a large refractive index nB11b and a large refractive index nB31a, and by satisfying the above conditions, symmetry is further improved, and a lens system capable of correcting aberrations well can be provided.

Another aspect of the present invention is an imaging apparatus including the lens system described above and an imaging device disposed on an image plane side of the lens system.

Drawings

Fig. 1 is a diagram showing a configuration of a lens system of embodiment 1, and (a) of fig. 1 shows a lens arrangement in which a focal position is infinity, and (b) of fig. 1 shows a lens arrangement in which a focal position is shortest.

Fig. 2 is a diagram showing data of each lens constituting the lens system of example 1.

Fig. 3 is a graph showing respective numerical values of the lens system of example 1.

Fig. 4 is a graph showing respective aberrations and MTF at the focal point at infinity in the lens system of embodiment 1.

Fig. 5 is a graph showing various aberrations and MTFs with the focal point in the middle of the lens system of embodiment 1.

Fig. 6 is a graph showing each aberration and MTF at the shortest focal point of the lens system of embodiment 1.

Fig. 7 is a diagram showing the structure of a lens system of embodiment 2, and (a) of fig. 7 shows a lens arrangement in which a focal position is infinity, and (b) of fig. 7 shows a lens arrangement in which a focal position is shortest.

Fig. 8 is a diagram showing data of each lens constituting the lens system of example 2.

Fig. 9 is a graph showing respective numerical values of the lens system of example 2.

Fig. 10 is a graph showing respective aberrations and MTF at the focal point at infinity in the lens system of example 2.

Fig. 11 is a graph showing each aberration and MTF with the focus in the middle of the lens system of embodiment 2.

Fig. 12 is a graph showing each aberration and MTF at the shortest focal point of the lens system of example 2.

Fig. 13 is a diagram showing the structure of a lens system of embodiment 3, and (a) of fig. 13 shows a lens arrangement in which a focal position is infinity, and (b) of fig. 13 shows a lens arrangement in which a focal position is shortest.

Fig. 14 is a diagram showing data of each lens constituting the lens system of example 3.

Fig. 15 is a graph showing respective numerical values of the lens system of example 3.

Fig. 16 is a graph showing respective aberrations and MTF at the focal point at infinity in the lens system of example 3.

Fig. 17 is a graph showing each aberration and MTF with the focus in the middle of the lens system of embodiment 3.

Fig. 18 is a graph showing each aberration and MTF at the shortest focal point of the lens system of example 3.

Fig. 19 is a diagram showing the configuration of a lens system of example 4, where (a) of fig. 19 shows a lens arrangement in which a focal position is infinity, and (b) of fig. 19 shows a lens arrangement in which the focal position is shortest.

Fig. 20 is a diagram showing data of each lens constituting the lens system of example 4.

Fig. 21 is a graph showing respective numerical values of the lens system of example 4.

Fig. 22 is a graph showing respective aberrations and MTFs at infinity in the focal point of the lens system of example 4.

Fig. 23 is a graph showing each aberration and MTF with the focus in the middle of the lens system of example 4.

Fig. 24 is a graph showing each aberration and MTF at the shortest focal point of the lens system of example 4.

Fig. 25 is a diagram showing a configuration of a lens system of example 5, and (a) of fig. 25 shows a lens arrangement in which a focal position is infinity, and (b) of fig. 25 shows a lens arrangement in which a focal position is shortest.

Fig. 26 is a diagram showing data of each lens constituting the lens system of example 5.

Fig. 27 is a graph showing respective numerical values of the lens system of example 5.

Fig. 28 is a graph showing respective aberrations and MTF at the focal point at infinity in the lens system of example 5.

Fig. 29 is a graph showing each aberration and MTF with the focus in the middle of the lens system of example 5.

Fig. 30 is a graph showing each aberration and MTF at the shortest focal point of the lens system of example 5.

Fig. 31 is a diagram showing the structure of a lens system of example 6, and (a) of fig. 31 shows a lens arrangement in which a focal position is infinity, and (b) of fig. 31 shows a lens arrangement in which a focal position is shortest.

Fig. 32 is a diagram showing data of each lens constituting the lens system of example 6.

Fig. 33 is a graph showing respective numerical values of the lens system of example 6.

Fig. 34 is a graph showing respective aberrations and MTFs at infinity in the focal point of the lens system of example 6.

Fig. 35 is a graph showing each aberration and MTF with the focus in the middle of the lens system of example 6.

Fig. 36 is a graph showing each aberration and MTF at the shortest focal point of the lens system of example 6.

Fig. 37 is a diagram showing the structure of a lens system of example 7, and (a) of fig. 37 shows a lens arrangement in which a focal position is infinity, and (b) of fig. 37 shows a lens arrangement in which a focal position is shortest.

Fig. 38 is a diagram showing data of each lens constituting the lens system of example 7.

Fig. 39 is a graph showing respective numerical values of the lens system of example 7.

Fig. 40 is a graph showing respective aberrations and MTFs at infinity in the focal point of the lens system of example 7.

Fig. 41 is a graph showing each aberration and MTF with the focus in the middle of the lens system of example 7.

Fig. 42 is a graph showing each aberration and MTF at the shortest focal point of the lens system of example 7.

Fig. 43 is a diagram showing the structure of a lens system of example 8, and (a) of fig. 43 shows a lens arrangement in which a focal position is infinity, and (b) of fig. 43 shows a lens arrangement in which a focal position is shortest.

Fig. 44 is a diagram showing data of each lens constituting the lens system of example 8.

Fig. 45 is a graph showing respective numerical values of the lens system of example 8.

Fig. 46 is a graph showing respective aberrations and MTF at the focal point at infinity in the lens system of example 8.

Fig. 47 is a graph showing each aberration and MTF with the focus in the middle of the lens system of example 8.

Fig. 48 is a graph showing each aberration and MTF at the shortest focal point of the lens system of example 8.

Detailed Description

Fig. 1 shows an example of an imaging device (camera, video camera device) including an optical system for imaging. Fig. 1 (a) shows a state of focusing on infinity, and fig. 1 (b) shows a state of focusing on the closest distance. The video camera (image pickup apparatus) 1 has a lens system (optical system, image pickup optical system, imaging optical system) 10 and an image pickup element (image pickup device, image plane, imaging plane) 5 arranged on an image plane side (image plane side, image pickup side, imaging side) 12 of the lens system 10. The lens system 10 for imaging is composed of a first lens group G1 of positive refractive power, a second lens group G2 of positive refractive power, and a third lens group G3 of positive refractive power, which are disposed in this order from the object side 11, and are disposed so as to sandwich the stop St with the first lens group G1. The first lens group G1, the stop St, and the second lens group G2 move integrally at the time of focusing, and the third lens group G3 is fixed at the time of focusing. That is, the distance between the third lens group G3 and the image plane 5 does not vary due to focusing.

The lens system 10 is a positive-positive 3-group structure and is a standard type lens system having a focal length of 55mm when subjected to 35mm conversion, and the first lens group G1 and the second lens group G2 move along the optical axis 7 upon focusing. Of the types in which the object side 11 is provided with positive power, a telephoto type (telephoto type) in which negative power is provided on the image plane side 12 is a general case, and a compact lens system can be provided in a standard type to the telephoto type. On the other hand, in order to diffuse the light flux converged by the lens having positive power on the object side to the image plane by the negative power on the rear side, the amount of refraction of the light beam at each lens becomes large, and particularly the amount of refraction at the lens on the object side where the positive power is concentrated becomes large, and it is difficult to perform aberration correction. In order to perform the aberration correction satisfactorily, many surfaces are required, the number of lenses tends to increase, and if the number of lenses increases, the influence of individual differences or tolerances becomes large. Further, if the number of lenses is increased, the MTF (modulation transfer function) tends to be decreased, and thus, although the MTF is increased in design, if the number of lenses is not increased as much as a predetermined accuracy at a predetermined position, the MTF is likely to be decreased or deteriorated.

In the lens system 10 of this example, the power arrangement of the telephoto type, positive-negative, is a 3-group configuration of positive powers, positive-positive, and the positive powers are dispersed in the lens groups G1 to G3. This makes it possible to avoid concentration of positive power in any one lens group, particularly the lens group on the object side 11, and to suppress the occurrence of aberration, and to perform aberration correction with a small number of lenses. Further, the third lens group G3 closest to the image plane side 12 has a lower positive power than the other lens groups on the object side 11, thereby providing a configuration suitable for the intermediate telephoto, and a configuration more suitable for aberration correction by combining lenses having negative powers as necessary.

Therefore, the combined focal length f3 of the third lens group and the combined focal length f12 of the first lens group and the second lens group may satisfy the following condition (1).

2≤f3/f12≤200…(1)

The lower limit of the condition (1) may be 3, and may be 100, and the upper limit may be 170. Therefore, the condition (1) may be the following condition (1 a).

3≤f3/f12≤200…(1a)

In particular, the range of the following condition (1b) can suppress occurrence of aberrations in the third lens group G3, being suitable for improvement of MTF. The lower limit of the condition (1b) may be 110. As described above, the upper limit may be 200.

100≤f3/f12≤170…(1b)

In addition, the range of the following condition (1c) can comparatively compactly group the third lens group G3, so that the lens system 10 can be provided compact as a whole. The upper limit of the condition (1c) may be 6.

2≤f3/f12≤10…(1c)

In the lens system 10 of this example, the third lens group G3 is formed by using the cemented lens B31 in which the cemented lens B31 includes a combination of the cemented surface S14 having a certain degree of curvature and the surfaces S13 and S15 having a large curvature and capable of securing a distance from the cemented surface S14, and the cemented lens B31 is used to correct each aberration including chromatic aberration without adjusting the surface interval. Therefore, in the third lens group G3, the ratio occupied by the cemented lens B31 is very large. On the other hand, if the distance (length) B31L of the cemented lens B31 is too large, the total length WL of the lens system 10 is too long, and the curvature of the cemented surface S14 is also too large, so that the manufacturing cost increases. Therefore, the cemented lens B31 of the third lens group G3 is made of glass having a high refractive index, and the cemented lens B31 having a predetermined aberration correction capability is compactly mounted.

Therefore, the distance on the optical axis 7 (the total length of the third lens group G3) G3L from the surface S13 closest to the object side 11 to the surface S17 closest to the image surface side 12 of the third lens group G3, and the distance on the optical axis 7 from the cemented lens B31 (the distance between the surface S13 and the surface S15, the length of the cemented lens B31) B31L may satisfy the following condition (2).

0.6≤B31L/G3L≤1…(2)

The lower limit of the condition (2) may be 0.65 and the upper limit may be 0.80, and particularly, the range of the following condition (2a) is suitable for the improvement of MTF.

0.65≤B31L/G3L≤0.8…(2a)

In addition, the refractive index nB31ab of at least one of the lens L31 with positive refractive power and the lens L32 with negative refractive power, which are combined with the lens B31, may satisfy the following condition (3).

1.8≤nB31ab≤2.0…(3)

Further, the distance (total length) G3L between the third lens group and the total length WL of the lens system 10 (the distance on the optical axis 7 from the surface S1 closest to the object side 11 to the surface S17 closest to the image side 12) may satisfy the following condition (4).

0.1≤G3L/WL≤0.5…(4)

The lower limit of the condition (4) may be 0.2, 0.25 or 0.28. The upper limit of the condition (4) may be 0.4.

Further, since the positive power of the third lens group G3 can be made small, it is effective to correct each aberration including chromatic aberration to dispose the lens L33 having negative power on the image plane side 12 of the cemented lens B31. It is effective that the third lens group G3 includes a cemented lens B31 disposed from the object side 11 and a lens L33 behind the negative refractive power recessed on the object side 11. By disposing the negative refractive power lens L33 depressed on the object side 11 at the position closest to the image plane side 12, a telephoto type or a configuration close to a telephoto type can be realized by a combination with the positive power lens groups G1 and G2 in front. Thus, the total length WL of the lens system is easily shortened. Further, the light flux toward the image plane 5 can be expanded by the negative power lens L33 closest to the image plane side 12, and a large image circle, for example, a size of about 55mm in diameter can be secured.

Further, by configuring the third lens group G3 to have a structure in which the lens B31 and the negative power lens L33 are joined, the power of each lens can be secured without increasing the power of the third lens group G3. Therefore, the aberration can be corrected more favorably without increasing the number of lenses so much, and the MTF can be easily improved.

The combined focal length f31ab of the positive refractive power lens L31 and the negative refractive power lens L32 in the combined lens B31 and the focal length f3GL of the rear lens L33 may satisfy the following condition (5).

0.1≤|f31ab/f3GL|1.1…(5)

The lower limit of the condition (5) may be 0.7 or 1.0. In particular, in the range of the following condition (5a), the power of the rear negative lens L33 is slightly higher than the power of the cemented lens B31, and aberrations can be corrected well.

0.1≤f31ab/f3GL|1.1…(5a)

In addition, the combined focal length f31ab of the positive refractive power lens L31 and the negative refractive power lens L32, the focal length f3GL of the rear lens L33, and the combined focal length f3 of the third lens group G3 in the joint lens B31 may satisfy the following condition (6).

0＜(|f31ab|+|f3GL|/|f3|≤1.3…(6)

As for the power of the third lens group G3, the total of the positive power and the negative power constituting the third lens group G3 can be made the same or larger, so that a structure suitable for aberration correction without increasing the power of the third lens group G3 can be provided. The upper limit of the condition (6) may be 1.0, 0.7, or 0.1. In particular, within a range satisfying the following condition (6a), the positive power of the cemented lens B31 and the negative power of the rear negative lens L33 are substantially equal, can be formed sufficiently large with respect to the power of the third lens group G3, and the total power of the third lens group G3 can be set weak, and thus are suitable for aberration correction.

0＜(|f31ab|+|f3GL|/|f3|≤0.1…(6a)

It is desirable that the cemented lens B31 of the third lens group G3 is a combination of a lens L31 of positive power and a lens L32 of negative power arranged from the object side 11. In the positive-positive lens system 10, the cemented lens B11 composed of a combination of the negative lens L11 and the positive lens L12 is disposed at the most object side 11 position of the first lens group G1, and a combination of symmetric powers is present at a position symmetric to the cemented lens B11, whereby the symmetry can be improved, and it is also effective for reduction of petzval sum. In the cemented lens B31, the refractive index nB31a of the positive power lens L31 having a large distance (thickness) along the optical axis 7 is desirably large, and the refractive index nB31a may satisfy the following condition (7).

1.65≤nB31a≤2.0…(7)

In addition, the following conditions (7a) and (11) may be satisfied in combination with the refractive index nB11B of the lens L12 that constitutes the positive power of the cemented lens B11 in a symmetrical position.

1.75≤nB31a≤2.0…(7a)

1.75≤nB11b≤2.0…(11)

In the cemented lens B31, as shown in condition (3), it is desirable that at least one of the refractive index nB31a of the positive power lens L31 and the refractive index nB31B of the negative power lens L32 has a refractive index of 1.8 or more. Thus, the refractive index nB31a of the lens L31 of which positive power is desired to be large may be the refractive index nB31a satisfying the following condition (7 b).

1.8≤nB31a≤2.0…(7b)

Since the cemented lens B31 can be thinned while maintaining a sufficient surface interval, the joining condition (2a) is suitable for disposing the negative power lens L33 behind the cemented lens B31 (on the image plane side 12).

Further, the refractive index nB31a of the lens L31 that is joined to the positive refractive power in the lens B31 may satisfy the following condition (7c) and may also satisfy the condition (7 d).

1.85≤nB31a≤2.0…(7c)

1.88≤nB31a≤2.0…(7d)

The power of the cemented surface S14 that is concave on the object side 11 of the cemented lens B31 can be easily obtained, and the aberration correction capability of the cemented lens B31 can be improved. Therefore, the number of lenses having a high refractive index constituting the lens system 10 can be reduced, and the lens system is economical.

In particular, it may be so that the refractive index nB31a of the positive power lens L31 in the cemented lens B31 is larger than the refractive index nB31B of the negative power lens L32, and the following condition (8) is satisfied.

0.5＜nB31b/nB31a＜1…(8)

The power of the cemented surface S14 that is concave on the object side 11 of the cemented lens B31 can be easily obtained, and the aberration correction capability of the cemented lens B31 can be improved. Therefore, the number of lenses having a high refractive index constituting the lens system 10 can be reduced, and the lens system is economical.

When attention is paid to the refractive index nB31B of the lens L32 that engages a negative power in the lens B31, the following condition (9) can also be satisfied.

1.60≤nB31b＜1.87…(9)

Further, the relationship between the refractive index nB31B of the negative power lens L32 in the cemented lens B31 and the refractive index n3GL of the rear negative power lens L33 in the third lens group G3 may satisfy the following condition (10).

0.5＜n3GL/nB31b＜1…(10)

By making the refractive index n3GL of the negative lens L33 concave on the object side 11 adjacent to the rear side (image surface side) 12 of the cemented lens B31 relatively small, the curvature of the object side 11 surface S16 of the negative lens L33 can be made large (the radius of curvature is made small). Therefore, the peripheral portion (edge portion) of the surface S16 of the negative lens L33 recessed on the object side 11 can be brought into close proximity to or into contact with the surface S15 on the image plane side 12 of the cemented lens B31, and the distance between the surface S15 and the surface S16 can be set. Therefore, the lens system 10 can be easily assembled, and the lens system 10 having a stable MTF and a good value can be provided.

The first lens group G1 may include a first cemented lens B11 including a negative refractive power lens L11 and a positive refractive power lens L12, and a second cemented lens B12 including a positive refractive power lens L13 and a negative refractive power lens L14, which are arranged in this order from the object side 11, the second lens group G2 may include a third cemented lens B21 including a negative refractive power lens L21 and a positive refractive power lens L22, which are arranged in this order from the object side 11, and a rear positive refractive power lens L23, the third lens group G3 may include a fourth cemented lens B31, and the fourth cemented lens B31 may include a positive refractive power lens L31 and a negative refractive power lens L32, which are arranged in this order from the object side 11.

The lens system 10 is configured to have substantially symmetrical power such as a lens having negative-positive-negative and a lens having negative-positive-negative so as to sandwich the diaphragm St from the object side 11, negative-positive and positive-negative cemented lenses B11 and B12 are configured from the diaphragm St to the object side 11, and negative-positive and positive-negative cemented lenses B21 and B31 are configured from the diaphragm St to the image plane side 12, and is also configured to be symmetrical in units of cemented lenses. The two cemented lenses B11 and B12 on the object side 11 are a combination of a positive meniscus type lens convex on the object side 11 and a negative meniscus type lens convex on the object side 11, and the two cemented lenses B21 and B31 on the image plane side 12 are a combination of a negative meniscus type lens concave on the object side 11 and a positive meniscus type lens concave on the object side 11, and in this regard, the orientations of the planes are also symmetrically arranged so as to sandwich the diaphragm St. Therefore, the arrangement is suitable for high symmetry, easy aberration correction, and reduction of petzval sum. Therefore, the lens system 10 can obtain a clear and bright image, and the MTF is easily improved.

In addition, by disposing the negative meniscus type cemented lenses B12 and B21 so as to face each other with the diaphragm St interposed therebetween, it is possible to pass a light flux parallel to the optical axis 7 through the diaphragm St, and it is possible to provide the lens system 10 which is brighter and has a small F value.

Further, the lens element has a 10-piece structure including the negative lens L33 closest to the image plane side 12, but includes 4 cemented lenses B11, B12, B21, and B31, and thus has 6 pieces as a lens element at the time of assembly. Therefore, the lens system 10 can be easily assembled, the positions of the 10 lenses (L11 to L14, L21 to L23, and L31 to L33) can be set with high accuracy, the MTF can be prevented from being degraded or reduced by assembly, and the lens system 10 with small tolerance variation by assembly and low assembly sensitivity (variation in performance by assembly) can be provided.

Further, the refractive index nB11B of the positive refractive power lens L12 in the first cemented lens B11 and the refractive index nB31a of the positive refractive power lens L31 in the fourth cemented lens B31 may satisfy the above-described conditions (7a) and (11). The powers of the joint surface S2 convex on the object side 11 and the joint surface S14 concave on the object side 11 of the joint lenses B11 and B31 located on the most object side 11 and the most image surface side 12 of the lens system 10 can be ensured. Therefore, aberration correction can be performed satisfactorily, and the number of high-refractive-index lens pieces included in the lens system 10 can be reduced, so that the low-cost and high-performance lens system 10 can be provided.

Further, the refractive index nB31a of the positive refractive power lens L31 in the cemented lens (fourth cemented lens) B31 of the third lens group G3 and the refractive index nB31B of the negative refractive power lens L32 in the fourth cemented lens B31 may satisfy the above condition (8), and the abbe number ν B11a of the negative refractive power lens L11 in the cemented lens (first cemented lens) B11 of the object side 11 of the first lens group G1 and the abbe number ν B11B of the positive refractive power lens L12 in the first cemented lens B11 may satisfy the following condition (12).

0.5＜vB11a/vB11b＜…(12)

By combining the negative-positive cemented lens B11 disposed closest to the object side 11 and the positive-negative cemented lens B31 disposed closest to the image plane side 12 of the lens system 10 with a positive lens having a high refractive index of, for example, 1.8 or more and low dispersion (high abbe number) and a negative lens having a low refractive index of, for example, 1.7 or less and high dispersion (low abbe number), it is possible to make the cemented surface S2 protruding toward the object side 11 closest to the object side 11 and the cemented surface S14 recessed toward the object side 11 closest to the image plane side 12 have optically symmetrical performance. Therefore, aberration correction can be performed satisfactorily, and the number of high-refractive-index lens pieces included in the lens system 10 can be reduced, so that the low-cost and high-performance lens system 10 can be provided.

The lens system 10 has a positive-positive 3-group structure, and the combined focal length f1 of the first lens group, the combined focal length f2 of the second lens group, and the combined focal length f3 of the third lens group may satisfy the following condition (13).

f2＜f1＜f3…(13)

By suppressing the power of the first lens group G1 disposed closest to the object side 11, it is possible to suppress the occurrence of aberration in the lens group in which the angle of light rays on the object side 11 is most likely to increase. Further, by employing a lens made of anomalous dispersion glass in the second lens group having the largest refractive power, it is effective for improving the performance (MTF) of the lens system 10 and also effective for correcting chromatic aberration. Therefore, at least one of the lenses L21 to L23 constituting the second lens group G2 may be made of an abnormally low dispersion glass. The second lens group G2 may include at least two lenses made of anomalous low dispersion glass. Specifically, the lens L22 of positive refractive power among the cemented lens (third cemented lens) B21 of the second lens group G2 may be an abnormally low dispersion glass. Further, the lens L23 of the second lens group G2, which is the positive refractive power behind the cemented lens B21, may be made of anomalous low dispersion glass.