JPH0694996A - Wide angle zoom lens having stationary group having positive refracting power - Google Patents

Abstract

PURPOSE:To provide the wide angle zoom lens which has a wide variable power ratio of about 2, is bright and compact by holding an exit pupil position apart from an image pickup plane and concentrating a strong positive refracting power to the object side of a second lens group. CONSTITUTION:This wide angle zoom lens is constituted, successively from an object side, of three lens groups; a first lens group I having a negative refracting power, a second lens group II having a positive refracting power and a third lens group III having a positive refracting power. The second lens group II moves forward and backward on the optical axis, thereby varying the power. The first lens group I moves in association with the second lens group II and corrects the movement of the focus position according to the variable power. The third lens group III is held fixed during the power variation. Conditions 4<f3/fw<10 are satisfied. In the equation, f3 is the focal length of the third lens group III, fw denotes the focal length at the wide angle end of the entire system. An aperture diaphragm is fixed between the first lens group I and the second lens group II during the power variation and is thereby disposed. The diaphragm position exists at 1.0 just before the second lens group II at the telephoto end.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compact wide-angle zoom lens, and more particularly to a telecentric zoom lens.

[0002]

2. Description of the Related Art In recent years, electronic still cameras and video cameras using solid-state image pickup devices such as CCD and MOS have become widespread. However, the exit pupil position of the optical system of these cameras can be sufficiently large from the image plane. Required. This is because the color filter on the image sensor is located slightly away from the image plane, so that when the light beam is obliquely incident, the aperture efficiency is substantially reduced, and the moire phenomenon is caused by the periodic structure of the image sensor. This is because it is required that the effective thickness of the crystal filter for preventing this does not fluctuate much on and around the axis. Further, some recent high-sensitivity small-sized image pickup devices have a microlens array immediately in front of the image pickup surface, and even in this case, if the exit pupils are not sufficiently separated, the aperture efficiency is lowered in the periphery.

Conventionally, the zoom lens system is composed of a first lens group having a negative refractive power and a second lens group having a positive refractive power in order from the object side, and zooming is performed by changing the interval between these groups. A lens having a relatively wide angle and a zoom ratio of about 2 is well known as a so-called two-group zoom. Many of these have their aperture stops placed in front of or behind the second lens group and move integrally with the second lens group during zooming. Such a zoom lens is used for CCD or MOS
When it is used in a camera using a solid-state image sensor such as the above, the position of the exit pupil is too close to the image pickup surface, which is not preferable.

Therefore, it is conceivable to place the fixed lens unit having a positive refractive power behind the second lens unit to move the exit pupil position away. As a zoom lens having a configuration in which such a positive fixed third lens group is added, there are disclosed in JP-A-60-120311, JP-A-60-181717, and JP-A-62-8.
Zoom lenses described in Japanese Patent No. 7925, Japanese Patent Laid-Open No. 2-118509, etc. are known. However, these are all designed mainly for single-lens reflex steel cameras, and due to the restrictions on the diameter of the lens mount and the compactness, the positive refractive power of the third group is extremely weak, and the exit pupil is sufficiently far away. This tendency becomes remarkable especially on the wide-angle side where the diaphragm position approaches the image plane together with the second lens group due to zooming, and it cannot be used for the above purpose.

[0005]

In view of such a situation, the present invention provides a wide-angle zoom lens having a variable power ratio of about 2 times, which is bright, compact, and has an exit pupil position sufficiently displaced from the imaging surface. It is intended to provide a zoom lens suitable for a camera or a video camera.

[0006]

In order to solve the above-mentioned problems, the basic construction of the zoom lens of the present invention comprises, in order from the object side, a first lens group having a negative refractive power and a positive refractive power. The second lens group has a second lens group and a third lens group having a positive refractive power, and the second lens group moves back and forth on the optical axis to perform zooming. Two
It is characterized in that it moves in relation to the lens group to correct the movement of the focal position due to zooming, and the third lens group is fixed during zooming and satisfies the following conditions. 4 <f ₃ / fw <10 (1) where f ₃ is the focal length of the third lens group, and fw is the focal length at the wide-angle end of the entire system.

An aperture stop is fixed between the first lens group and the second lens group during zooming.

More specifically, in the zoom lens according to the present invention, the second lens group includes, in order from the object side, at least two positive lenses, one negative lens with a strong concave surface facing the image side, and one positive lens. Of the at least two positive lenses that are composed of lenses and are on the object side, one may be a cemented doublet consisting of a positive lens and a negative lens, and the following conditions are satisfied. 0.65 <| m (2T) | <0.8 (m (2T) <0) (2) 1.65 <np (3) However, m (2T) is an image at the telephoto end of the second lens group. The magnification, np, is the average value of the refractive indices of the two positive lenses closest to the object in the second lens group. When the positive lens is composed of a cemented doublet, np is the refractive index of the positive lens that constitutes it.

Further, the first lens group is composed of, in order from the object side, a meniscus lens having a negative refractive power with a convex surface facing the object side, a negative lens, and a meniscus lens having a positive refractive power having a convex surface facing the object side. It

[0010]

In the zoom lens of the present invention, the positive refractive power of the third lens unit fixed during zooming is made stronger than that of the conventional one, and the diaphragm position is between the first lens unit and the second lens unit. The position of the exit pupil is kept away from the image pickup surface by fixing it at, and it has a remarkable effect especially at the wide-angle end. In addition, the back focus was shortened by concentrating a strong positive refracting power on the object side of the second lens group, and the overall length was made compact. Further, in order to realize a bright aperture ratio as compared with the conventional one, a high refractive index glass material is used for the positive lens of the second lens group in which the axial light flux is highest.

The condition (1) relates to the refracting power of the third lens group. If the upper limit is exceeded, the exit pupil position cannot be set sufficiently far, and if the lower limit is exceeded, it is advantageous for the exit pupil position. The refracting power arrangement of the system becomes unbalanced, and negative distortion occurs significantly at the wide-angle end.

The condition (2) relates to the back focus.
If the upper limit of this condition is exceeded, the back focus becomes too long at the telephoto end, and the compactness becomes unsatisfactory. If the lower limit is exceeded, the total length will become shorter at the telephoto end, but with this,
Since the predetermined focal length of the entire system is secured at the telephoto end, the refracting power of the first lens group becomes weak and the movement amount of the first lens group increases.

The condition (3) relates to the refractive indices of the two positive lenses on the object side in the second lens unit. If this condition is not satisfied,
Spherical aberration is undercorrected at the wide-angle end where the F number is relatively bright.

By fixing the aperture position during zooming,
The diaphragm device, which is a member having a housing portion for the diaphragm blade and larger than the lens diameter, does not move during zooming, which is a great advantage in terms of the lens frame structure.

[0015]

EXAMPLES Examples of the zoom lens of the present invention will be shown below. In any of the embodiments, the diaphragm position is at the telephoto end and 1.0 immediately before the second lens group. The display of the exit pupil position is based on the image plane and has a minus sign when measured toward the object side. In the table, R is the radius of curvature of each lens surface, D is the lens center thickness or lens spacing, N is the refractive index of the glass material, νd is the Abbe number, f is the focal length of the entire lens system,
2ω is the angle of view, F is the F number, fB is the back focus, Y is the maximum image height, and A and B are the movement amounts of the second lens group.

Example 1 f = 12.50 to 24.25 F: 2.70 to 3.60 2ω = 54.9 to 30.0 Y = 6.50 No. RD N νd 1 19.550 1.00 1.83400 37.2 2 10.913 4.50 3 1st lens group 120.915 1.00 1.79952 42.2 4 21.792 0.20 5 14.592 2.50 1.80518 25.4 6 33.681 A 7 26.029 2.00 1.77250 49.6 8 -48.290 0.20 9 2nd lens group 8.511 3.00 1.77250 49.6 10 10 20.792 0.50 11 76.839 1.50 6.805 1.805 2.50 13 1012.457 2.00 1.71300 53.9 14 -22.557 B 15 3rd lens group -33.220 2.00 1.71300 53.9 16 -20.008 3.88 17 Cover glass ∞ 5.00 1.51633 64.1 18 ∞ Variable spacing and exit pupil position f A B Exit pupil position 12.50 23. 98 2.30 -50 16.94 13.14 5.17 -43 24.25 4.00 9.99 -42 focal length _{f 1 = -32.895 f 2 = 18.310} f 3 of the lens groups _{= 66.375 f 3 /fw=5.31 m (2T} ) = 0.739 np = 1.7725

Example 2 f = 10.80 to 20.15 F: 2.80 to 4.00 2ω = 62.1 to 35.2 Y = 6.50 No. RD N νd 1 21.884 0.90 1.83400 37.2 2 12.019 3.50 3 1st lens group 52.330 0.90 1.83400 37.2 4 17.495 1.40 5 15.112 2.50 1.84666 23.8 6 30.960 A 7 71.753 2.00 1.71300 53.9 8 -81.294 0.20 9 2nd lens group 17.028 2.00 1.71300 53.9 300 10 -334.973 0.20 ll 5126 2.00 1.71 13.518 0.70 13 36.586 2.00 1.84666 23.8 14 6.376 1.48 15 -85.465 2.00 1.71300 53.9 16 -16.883 B 17 3rd lens group -30.139 2.00 1.71300 53.9 18 -17.702 3.97 19 Cover glass ∞ 6.00 1.51633 64.1 20 ∞ Variable distance and exit pupil position f a B exit pupil position 10.80 23.21 1.00 -39 15.01 11.30 3.73 -36 20.51 3.00 7.00 -35 focal length f ₁ of the lens groups = - _{2.505 f 2 = 16.727 f 3 =} 56.394 f 3 /fw=5.222 m (2T) = 0.741 np = 1.713

Example 3 f = 10.80 to 20.52 F: 2.80 to 4.00 2 ω = 62.1 to 35.2 Y = 6.50 No. RD N νd 1 20.142 0.90 1.83400 37.2 2 11.717 3.50 3 1st lens group 70.102 0.90 1.83400 37.2 4 15.569 1.40 5 15.165 2.50 1.84666 23.8 6 41.475 A 7 12.544 3.00 1.78590 44.2 8 -17.065 1.20 1.84666 23.8 9 2nd lens group -40.823 0.20 910 9.622 2.00 1.83400 37.2 12 19.449 0.60 -34.854 2.00 1.84666 23.8 13 13 6.862 1.20 14 -190.125 2.00 1.59270 35.3 15 15 -11.861 B 16 3rd lens group -33.313 2.00 1.64769 33.8 17 -22.304 3.07 18 Cover glass ∞ 6.00 1.51633 64.1 19 ∞ Variable spacing and exit pupil position f A B exit pupil position 10.80 23.19 1.00 -30 15.01 11.27 3.39 -29 20.52 3.00 6.31 -29 focal length f of the lens group ₁ = -31.489 _{2 = 15.736 f 3 = 97.261 f} 3 /fw=9.01 m (2T) = 0.700 np = 1.80995

[0019]

As is apparent from the above-mentioned embodiments and aberration diagrams, the zoom lens of the present invention is relatively wide-angle, bright and compact with a zoom ratio of approximately 2 and has a sufficient exit pupil position. It was possible to configure the lens so that it was at a distance, and the aberration was well corrected.

[Brief description of drawings]

FIG. 1 is a sectional view of a zoom lens according to a first embodiment of the present invention at a middle position.

FIG. 2 is a sectional view of a zoom lens according to a second embodiment of the present invention in a middle position.

FIG. 3 is a sectional view of a zoom lens according to a third embodiment of the present invention in a middle position.

FIG. 4 is an aberration curve diagram of the first embodiment of the zoom lens according to the present invention.

FIG. 5 is an aberration curve diagram of a second embodiment of the zoom lens according to the present invention.

FIG. 6 is an aberration curve diagram of a third embodiment of the zoom lens according to the present invention.

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[Procedure amendment]

[Submission date] October 5, 1993

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

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Claims (2)

[Claims] 1. A three-lens group having, in order from the object side, a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power. , The second lens group moves back and forth on the optical axis to perform zooming, and the first lens group moves in relation to the second lens group to correct the movement of the focal position due to zooming, and the third lens group A wide-angle zoom lens characterized in that the group is fixed during zooming and satisfies the following conditions. 4 <f ₃ / fw <10 where f ₃ is the focal length of the third lens group, and fw is the focal length at the wide-angle end of the entire system. 2. The zoom lens according to claim 1, wherein an aperture stop is fixedly disposed between the first lens group and the second lens group during zooming.