JPH05157964A - Inner focus type telephoto lens - Google Patents

Abstract

PURPOSE:To provide the inner focus type telephoto lens which has high optical performance after aberration variation in focusing is excellently compensated by properly setting the lens constitution of the telephoto lens and a lens groups which are moved at the time of the focusing. CONSTITUTION:This telephoto lens has three lens groups which are a 1st group 1 with positive refracting power, a 2nd group 2 with negative refracting power, and a 3rd group 3 with positive refracting power in order from an object side; and the 1st group 1 consists of three positive lenses and a negative lens which is concave to an image plane side, the 3rd group 3 consists of a cemented lens composed of a negative and a positive lens and a biconvex positive lens, and the 2nd group 2 is moved on the optical axis for focusing and satisfies 2.2<f12/f<4.5, where f12 is the composite focal distance of the 1st and 2nd groups when the 2nd group is put in focus on an infinite-distance body and (f) is the focal distance of the whole system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inner focus type telephoto lens suitable for a photographic camera, a video camera or the like, and particularly, it corrects aberration fluctuation during focusing.
The present invention relates to an inner focus type telephoto lens having good optical performance with a focal length of 85 mm and an F number of about 1.8 when converted for a 5 mm film.

[0002]

2. Description of the Related Art Generally, focusing in a taking lens is performed by moving the entire taking lens or moving a part of the taking lens. If the taking lens is a telephoto lens having a long focal length, the taking lens becomes large and heavy, and it is mechanically difficult to move the taking lens as a whole to perform focusing.

For this reason, in many telephoto lenses, some of the lens groups are moved for focusing. Of these, various proposals have been made to use the inner focus type in which a part of the central lens group in the relatively small and lightweight lens system other than the front lens group of the taking lens is moved for focusing. ing. For example, JP-A-55-
In Japanese Patent No. 147606, an inner focus type telephoto lens having a focal length of 300 mm and an F number of 2.8 is used, and in Japanese Patent Laid-Open Nos. 59-65820 and 59-65821, a focal length of 135 mm and an F number of about 2.8. Has proposed an inner focus type telephoto lens.

In each of the inner focus type telephoto lenses proposed by these, the first group having a positive refractive power, the second group having a negative refractive power, and the third group having a positive refractive power are arranged in this order from the object side. It has three lens groups, and the second group is moved on the optical axis for focusing.

[0005]

In the inner focus type, the focusing lens group is small and lightweight, so that the operability is easy and high-speed operation is possible, and the lens when focusing on an object at infinity and a near object is used. There are advantages that the center of gravity of the whole system does not change and holding is easy. On the other hand, when the inner focus type is adopted, the fluctuation of aberration at the time of focusing becomes large, and it is difficult to satisfactorily correct the fluctuation of aberration at this time, which causes the deterioration of the optical performance.

The present invention satisfactorily corrects aberration fluctuations during focusing and converts it to a 35 mm film having good optical performance over the entire object distance, and has a focal length of 85 mm, F
An object of the present invention is to provide an inner focus type telephoto lens suitable for a photographic camera or a video camera having a number of about 1.8.

[0007]

The inner focus type telephoto lens of the present invention comprises a first group having a positive refractive power, a second group having a negative refractive power, and a third group having a positive refractive power in order from the object side. A cemented lens having three lens groups, the first group consisting of three positive lenses and a negative lens having a concave surface facing the image side, and the third group including a negative lens and a positive lens cemented to each other. And both lens surfaces are convex positive lenses, the second group is moved on the optical axis for focusing, and when the second group is focused on an object at infinity, the first group and the second group F is the combined focal length
12. When the focal length of the entire system is f, the condition is 2.2 <f12 / f <4.5 (1).

[0008]

1 to 3 are numerical examples 1 of the present invention,
2 and 3 are lens cross-sectional views, and FIGS. 4 to 6 are aberration diagrams of Numerical Examples 1, 2, and 3 of the present invention. In the aberration diagram (A)
Shows an object at infinity, and (B) shows an object distance of 0.8 m. In the figure, 1 is a first group having a positive refractive power, 2 is a second group having a negative refractive power, 3 is a third group having a positive refractive power, and SP is a diaphragm.

In this embodiment, the second group 2 is moved to the image plane side as indicated by the arrow to focus from an infinitely distant object to a close object.

In this embodiment, in order from the object side, the first lens unit has a positive first lens, a meniscus-shaped second lens having a convex surface facing the object side, and a meniscus-shaped positive lens having a convex surface facing the object side. The third lens is composed of four independent meniscus negative fourth lenses with the concave surface facing the image side.

Of these, the three positive lenses on the object side gradually converge the axial light beam so that spherical aberrations generated by the first to third lenses are reduced as much as possible. The positive spherical aberrations generated by the first to third lenses are corrected in good balance by the positive spherical aberrations generated by the concave lens surface on the image surface side of the fourth lens.

Further, the lens surface on the image side of the third lens and the fourth lens surface
The negative air lens formed by the lens surface on the object side of the lens mainly generates high-order negative spherical aberration, and the spherical aberration and sagittal flare of the entire system are well corrected.

The third lens group is composed of a cemented lens having a positive and weak refractive power as a whole in which a negative lens and a positive lens are cemented together, and a positive lens whose both lens surfaces are convex. This cemented lens uses a high-dispersion glass for the negative lens and a low-dispersion glass for the positive lens to correct chromatic aberration in the third group itself, and to reduce fluctuations in chromatic aberration when focusing in the second group. ing. The positive lens arranged on the image plane of the cemented lens bears most of the positive refracting power of the third lens group.
The positive distortion generated in the group is corrected in the negative direction.

In this embodiment, the negative lens and the positive lens of the cemented lens may be reversed, and the cemented lens may be constructed by cementing the positive lens and the negative lens.

Further, in this embodiment, the ratio of the combined focal length f12 of the first and second groups and the focal length f of the entire system when the second group is focused on an object at infinity is expressed by the above conditional expression. By setting as in (1), the aberration variation due to focusing when achieving a telephoto lens with a focal length of 85 mm and an F number of about 1.8 is favorably corrected while shortening the overall lens length.

If the combined focal length f12 of the first group and the second group becomes too short beyond the lower limit of the conditional expression (1), the telephoto type of the entire lens system is emphasized and the total lens length becomes short. The back focus becomes insufficient, and the aberration variation when focusing with the second lens group becomes large.

On the contrary, if the upper limit of conditional expression (1) is exceeded and the combined focal length of the first group and the second group becomes too long, the back focus becomes sufficiently long, but the total lens length also increases, and the lens system increases. It is not good because the whole becomes larger.

The inner focus type telephoto lens which is the object of the present invention can be achieved by satisfying the above-mentioned various conditions. In order to further shorten the total lens length and obtain high optical performance over the entire screen, the following various It is good to satisfy the conditions.

(A) The focal lengths of the second and third lens units are f
2 and f3, 0.55 <| f2 | / f <0.85 (2) 0.75 <f3 / | f2 | <1.15 (3) ..

Conditional expression (2) sets the ratio of the focal length of the second lens system to the focal length of the entire system appropriately, and mainly reduces the aberration variation when the second lens unit is moved to perform focusing. It is a thing. If the negative refracting power of the second lens unit becomes too strong beyond the lower limit of conditional expression (2), the amount of extension when focusing from an infinitely distant object to a close object decreases, but occurs from the second lens group with the focus. The various aberrations, especially the spherical aberration, increase, and the distortion also increases at the closest object distance, which is not good. If the upper limit of conditional expression (2) is exceeded and the negative refractive power of the second lens unit becomes too weak, the amount of extension at the time of focusing increases, and the overall lens length becomes long, which is not good.

Conditional expression (3) appropriately sets the ratio of the focal length of the third lens unit to the focal length of the second lens unit based on the conditional expression (2), and shortens the total lens length, while improving various aberrations. It is for correction. When the lower limit of conditional expression (3) is exceeded and the positive refracting power of the third lens unit becomes too strong as compared with the refracting power of the second lens unit, the telephoto type becomes too weak and the total lens length becomes long and Aberrations generated from the group increase. When the upper limit of conditional expression (3) is exceeded and the positive refractive power of the third lens unit becomes too weak, the total lens length becomes short, but the moving space of the second lens unit at the time of focusing becomes insufficient, and at the close object distance. This is not good because the distortion aberration increases in the positive direction.

(B) The aperture stop SP may be fixed between the first group and the second group, but particularly at a close object distance, lateral chromatic aberration is corrected well and optical performance when stopped is kept good. In order to do so, it is preferable to move it integrally with the second group, or to arrange it between the second group and the third group and move it integrally with the second group.

Next, numerical examples of the present invention will be shown. In the numerical examples, Ri is the radius of curvature of the i-th lens surface in order from the object side, Di is the i-th lens thickness and air gap from the object side, and Ni and νi are the values of the i-th lens in order from the object side, respectively. The refractive index of glass and the Abbe number. Table 1 shows the relationship between the above conditional expressions and numerical examples.

Numerical Example 1 F = 85.0 FNO = 1: 1.8 2ω = 28.6 ° R 1 = 185.36 D 1 = 4.40 N 1 = 1.65160 ν 1 = 58.5 R 2 = -427.66 D 2 = 0.20 R 3 = 63.58 D 3 = 4.60 N 2 = 1.77250 ν 2 = 49.6 R 4 = 156.32 D 4 = 0.15 R 5 = 33.99 D 5 = 8.20 N 3 = 1.71300 ν 3 = 53.8 R 6 = 241.98 D 6 = 0.16 R 7 = 278.09 D 7 = 3.20 N 4 = 1.71736 ν 4 = 29.5 R 8 = 26.41 D 8 = 8.82 R 9 = (Aperture) D 9 = Variable (3.59 ~) R 10 = -101.48 D10 = 2.40 N 5 = 1.80518 ν 5 = 25.4 R 11 = -52.90 D11 = 1.50 N 6 = 1.51633 ν 6 = 64.2 R 12 = 36.36 D12 = variable (16.3 ~) R 13 = -9058.59 D13 = 2.00 N 7 = 1.80518 ν 7 = 25.4 R 14 = 41.85 D14 = 12.40 N 8 = 1.80610 ν 8 = 41.0 R 15 = -410.77 D15 = 0.20 R 16 = 73.06 D16 = 5.40 N 9 = 1.77250 ν 9 = 49.6 R 17 = -122.05 (Numerical example 2) F = 85.0 FNO = 1: 1.8 2ω = 28.6 ° R 1 = 188.13 D 1 = 4.40 N 1 = 1.65160 ν 1 = 58.5 R 2 = -422.79 D 2 = 0.20 R 3 = 63.56 D 3 = 4.60 N 2 = 1.77250 ν 2 = 49.6 R 4 = 149.28 D 4 = 0.20 R 5 = 33.93 D 5 = 8.20 N 3 = 1.71300 ν 3 = 53.8 R 6 = 277.84 D 6 = 0.16 R 7 = 326.46 D 7 = 3.20 N 4 = 1.71736 ν 4 = 29.5 R 8 = 26.48 D 8 = 9.10 R 9 = (Aperture) D 9 = Variable (3.50 ~) R 10 = -107.52 D10 = 2.50 N 5 = 1.80518 ν 5 = 25.4 R 11 = -55.97 D11 = 1.50 N 6 = 1.51633 ν 6 = 64.2 R 12 = 36.67 D12 = Variable (18.17〜) R 13 = -409.61 D13 = 2.00 N 7 = 1.80518 ν 7 = 25.4 R 14 = -51.80 D14 = 7.00 N 8 = 1.80610 ν 8 = 41.0 R 15 = -226.42 D15 = 0.20 R 16 = 71.13 D16 = 5.40 N 9 = 1.77250 ν 9 = 49.6 R 17 = -126.92 (Numerical Example 3) F = 85.0 FNO = 1: 1.8 2ω = 28.6 ° R 1 = 253.22 D 1 = 4.40 N 1 = 1.65160 ν 1 = 58.5 R 2 = -290.11 D 2 = 0.20 R 3 = 63.55 D 3 = 4.60 N 2 = 1.77250 ν 2 = 49.6 R 4 = 156.23 D 4 = 0.20 R 5 = 35.19 D 5 = 8.20 N 3 = 1.71 300 ν 3 = 53.8 R 6 = 260.42 D 6 = 0.16 R 7 = 306.92 D 7 = 3.20 N 4 = 1.71736 ν 4 = 29.5 R 8 = 27.69 D 8 = 9.51 R 9 = (aperture) D 9 = Variable (3.41 ~) R 10 = -114.27 D10 = 2.50 N 5 = 1.80518 ν 5 = 25.4 R 11 = -60.32 D11 = 1.50 N 6 = 1.51633 ν 6 = 64.2 R 12 = 37.49 D12 = Variable (20.26 ~) R 13 = -218.77 D13 = 2.00 N 7 = 1.80518 ν 7 = 25.4 R 14 = -48.46 D14 = 7.00 N 8 = 1.83481 ν 8 = 42.7 R 15 = -158.09 D15 = 1.00 R 16 = 77.48 D16 = 5.40 N 9 = 1.83481 ν 9 = 42.7 R 17 = -153.57

[0025]

[Table 1]

[0026]

According to the present invention, each lens group of the telephoto lens is specified as described above, and the second group is moved on the optical axis to perform focusing, thereby shortening the total lens length and aberrations associated with the focusing. It is possible to achieve an inner focus type telephoto lens suitable for a photographic camera, a video camera, or the like, which has a high optical performance in which fluctuations are well corrected.

[Brief description of drawings]

FIG. 1 is a lens cross-sectional view of Numerical Example 1 of the present invention.

FIG. 2 is a lens cross-sectional view of Numerical Example 2 of the present invention.

FIG. 3 is a lens cross-sectional view of Numerical Example 3 of the present invention.

FIG. 4 is a diagram showing various aberrations of Numerical Example 1 of the present invention.

FIG. 5 is a diagram of various types of aberration in Numerical example 2 of the present invention.

FIG. 6 is a diagram showing various types of aberration in Numerical Example 3 of the present invention.

[Explanation of symbols]

 1 1st group 2 2nd group 3 3rd group SP Aperture ΔS Sagittal image plane ΔM Meridional image plane

Claims (1)

[Claims] 1. A three-lens group including a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power in order from the object side, and the first lens group. The third lens unit includes three positive lenses and a negative lens having a concave surface facing the image side, and the third group includes a cemented lens in which a negative lens and a positive lens are cemented together and a positive lens having convex lens surfaces on both sides. When the second group is moved on the optical axis for focusing and the second group is focused on an object at infinity, the combined focal length of the first group and the second group is f12, and the focal length of the entire system is f. An inner-focus telephoto lens characterized by satisfying the condition 2.2 <f12 / f <4.5.