JPH0478806A - Rear focus type zoom lens - Google Patents

Abstract

PURPOSE:To realize a large diameter ratio and high power variation, also to shorten the lens overall length, and to offer the rear focus type zoom lens which has excellent optical performance by composing the lens system of four lens groups and satisfying a specific condition. CONSTITUTION:The zoom lens has a 1st group with positive refracting power, a 2nd group with negative refracting power, a 3rd group with positive refracting power, and a 4th group with positive refracting power, i.e. four lens groups in order from the object side; and the 2nd group is moved toward the image plane for power variation from the wide-angle end to the telephoto end and the 4th group is moved to correct variation of the image plane due to the power variation and also put the lens in focus. The inequality I holds, where F1 is the focal length of an (i)th group and FT, FNOT, and omegaT are the focal length, open-aperture F number, and half view angle of the whole system at the telephoto end. Consequently, the rear focus type zoom lens is obtained suitably as a zoom lens which has a 6 large power variation ratio and an about 1.8 F number large diameter ratio and is used for, specially, a video camera, a still video camera, a camera for broadcasting, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a zoom lens of the rear focus type, which is particularly suitable for use in video cameras, still video cameras, broadcast cameras, etc., and has a zoom ratio of 6 and a large aperture ratio of approximately 1.8. The present invention relates to a focus-type zoom lens suitable for a multiplier zoom lens.

[Conventional technology]

Zoom lenses have traditionally been characterized by satisfying the zoom conditions of photographic cameras, video cameras, and the like.

(2) Various types of zoom lenses have been proposed that employ the Okas formula: 0.09<lF2/Fa)<0.2.

In general, rear-focus zoom lenses have a smaller effective diameter of the first group than zoom lenses that focus by moving the first group, making it easier to downsize the entire lens system, and also suitable for close-up photography, especially in extreme Close-up photography is facilitated, and since the relatively small and lightweight lens group is moved, the driving force for the lens group is small and quick focusing is possible.

For example, as such a rear-1 orcus type zoom lens, Japanese Patent Laid-Open No. 63-44614 discloses, in order from the object side, a first group with positive refractive power, a second group with negative refractive power for variable magnification, and a second group with negative refractive power for variable magnification. a third group with negative refractive power to correct image fluctuations associated with magnification;
In a so-called four-group zoom lens consisting of four lens groups including a fourth group having positive refractive power, focusing is performed by moving the third group. However, this zoom lens has a tendency to increase the overall length of the lens because it is necessary to secure a movement space for the third group.

In Japanese Unexamined Patent Publication No. 58-136012, a variable magnification section is composed of three or more lens groups, and focusing is performed by moving some of the lens groups.

In JP-A-63-247316, in order from the object side, a first group with positive refractive power, a second group with negative refractive power, a third group with positive refractive power, and a fourth group with positive refractive power. It has four lens groups, the second group is moved to perform magnification change, and the fourth group is moved to perform image plane fluctuation and focus accompanying magnification change.

JP-A-58-160913 discloses, in order from the object side, a first group with positive refractive power, a second group with negative refractive power, a third group with positive refractive power, and a fourth group with positive refractive power. It has four lens groups, the first group and the second group are moved to change the magnification, and the image plane changes due to the magnification change are caused by moving the fourth group. Focusing is performed by moving one or more of these lens groups.

[Problem that the invention is trying to solve]

In general, when a rear focus system is adopted in a zoom lens, the entire lens system can be made smaller as described above, rapid focusing is possible, and close-up photography is facilitated.

However, on the other hand, the problem is that aberration fluctuations during focusing become large, making it extremely difficult to achieve high optical performance while reducing the size of the entire lens system over a wide range of object distances, from objects at infinity to objects at close distances. will arise.

Particularly in the case of a zoom lens with a large aperture ratio and a high zoom ratio, it becomes very difficult to obtain high optical performance over the entire zoom range and over the entire object distance.

The present invention uses a rear focus system to increase the aperture ratio, increase the zoom ratio, and shorten the overall length of the lens. The purpose of the present invention is to provide a rear focus type zoom lens having good optical performance over the entire object distance.

〔Example〕

FIG. 1 is a schematic diagram of an embodiment showing the paraxial refractive power arrangement of a rear focus type zoom lens according to the present invention.

In the figure, ■ is the first group with positive refractive power, and ■ is the second group with negative refractive power.
The group ■ is the third group with positive refractive power, and ■ is the fourth group with positive refractive power. SP is an aperture stop, which is placed in front of the third group (2).

When changing the magnification from the wide-angle end to the telephoto end, the second group is moved toward the image plane as shown by the arrow, and image fluctuations caused by the zooming are corrected by moving the fourth group.

Additionally, a rear focusing system is adopted in which focusing is performed by moving the fourth group on the optical axis. The solid line curve 4a and the dotted line curve 4b of the fourth group shown in the figure represent image plane fluctuations when changing the magnification from the wide-angle end to the telephoto end when focusing on an object at infinity and a close object, respectively. It shows the movement trajectory for correction.

Note that the first group and the third group are fixed during zooming and focusing.

In this embodiment, the fourth group is moved to correct image fluctuations caused by zooming, and the fourth group is also moved to perform focusing. In particular, as shown by curves 4a and 4b in the figure, when changing the magnification from the wide-angle end to the telephoto end, the lens is moved so as to have a convex locus toward the object side. This makes effective use of the space between the third and fourth groups and effectively shortens the overall length of the lens.

In this embodiment, for example, when focusing from an object at infinity to a close object at the telephoto end, straight line 4C in the figure is used.
This is done by moving the fourth group forward as shown in the figure.

In this example, compared to a conventional 4-group zoom lens in which focusing is performed by extending the first group, the above-mentioned mirror focusing method is used to effectively prevent an increase in the effective diameter of the first group. ing.

Furthermore, in this embodiment, by constructing the third group with a single lens having an aspherical surface, the number of lenses can be reduced, and at the same time, spherical aberration and coma can be effectively corrected by the aspherical surface.

Furthermore, by introducing an aspheric surface into at least one lens group of the fourth group, off-axis astigmatism, field curvature, etc. are effectively corrected. By setting the above-mentioned conditional expression (1), it is possible to obtain a zoom lens that has good optical performance over the entire zoom range and object distance while reducing the size of the entire lens system.

Next, technical opinions regarding the above-mentioned conditional expression will be explained.

Conditional expression (1) is used to appropriately set the refractive power of the first group and to satisfactorily correct various aberrations while downsizing the entire lens system.

When the upper limit of conditional expression (1) is exceeded and the refractive power of the first group becomes weaker, aberration correction becomes easier, but the distance between the first group and the diaphragm increases, and it becomes difficult to secure off-axis light flux. The lens diameter of the first group increases. If the lower limit is exceeded and the refractive power of the first group becomes strong, the overall length of the lens will be shortened, but the distance between it and the second group will be shortened, which is not good because physical interference is likely to occur.

Furthermore, under the above conditions, the zoom lens according to the present invention satisfies 0.09<l F2/FT+<0.2 (
2) 0.59×IF3/F41<0.85...(3)
It is preferable to satisfy the following conditions in order to reduce fluctuations in aberration due to zooming and obtain good optical performance over the entire zooming range.

Conditional expression (2) relates to the refractive power of the second group, and is intended to effectively obtain a predetermined zoom ratio while reducing fluctuations in aberrations accompanying zooming. If the refractive power of the second group exceeds the lower limit and becomes too strong, it will be easier to downsize the entire lens system, but the Petzval sum will increase in the negative direction, the curvature of field will increase, and aberrations will fluctuate with zooming. is getting bigger. If the upper limit is exceeded and the refractive power of the second group becomes too weak, aberration fluctuations due to zooming will decrease, but the amount of movement of the second group to obtain a predetermined zoom ratio will increase, and the overall length of the lens will become longer. It's not good because it's getting worse.

Conditional expression (3) relates to the focal length of the third lens group and the fourth lens group, and is intended to maintain good optical performance while achieving compactness after the aperture.

If the focal length of the third lens group becomes short by exceeding the lower limit of conditional expression (3), it becomes difficult to correct fluctuations in spherical aberration accompanying zooming or during focusing.

arise.

Conversely, if the upper limit is exceeded and the focal length of the fourth lens group becomes short, it becomes difficult to shorten the overall lens length, and the angle of incidence of off-axis light on the fourth lens becomes large, making it difficult to correct aberrations in the fourth lens group. .

Next, numerical examples of the present invention will be shown. In numerical examples R
i is the radius of curvature of the i-th lens surface in order from the object side, D
i is the i-th lens thickness and air distance from the object side, Ni
and νl are the refractive index and Abbe number of the glass of the first lens, respectively, in order from the object side.

The aspherical shape has an X axis in the optical axis direction, an H axis in a direction perpendicular to the optical axis,
The traveling direction of the light is positive, R is the paraxial radius of curvature, AXB, C,
When D and E are each aspherical surface coefficients, it is expressed by the formula +DH"+EH".

Further, Table 1 shows the relationship with each conditional expression in each numerical example. Note that R18 and R19 in the numerical examples are glass materials such as a face plate.

Numerical Example 1 R7= R8= RIO= R18= R19= 8.4131 2.7100 15.6780 2.4161 0.7999 1.2229 1.2229 232.4816 F=]~57 D 7= D 8= D 9 = IO FNO = 1: 185 ~ 2.62 0.1414 0.5824 0.0333 0.3577 0.3796 0.0832 0.2829 Variable 1.0815 2ω = 56.1 ' ~ 10.7°N 1 =
1.80518 N 2= 1.60311 3= 1,80400 1,8830O N 5= 1.51742 N 6= 1.84666 N 7= N 8= N 9= NI O= 1.58313 1.84666 1.58313 1.51633 12th surface aspherical surface Ro= 1 Knee 4 17th surface aspherical surface Ro= 2 = 6 3994XIO-'7827XlO-" B: D= B= 11abxtu 1= 25.4 ν 2= 60.7 3 = 46.6 4= 40.8 ν 5= 52.4 ν 6= 23.8 ν 7- 8= 9= ν10= 59.4 23.8 64.1 Numerical Example 2 R14 R18= 10.3624 2.9200 10°6103 2.4791 7.9+21 17.0692 0.8441 2.5343 F=1~5.7 FNO=1 185~2.61 D 1.8 0.1393 0.5738 0.0328 0 .3525 Variable 0.0820 0.3624 0.0820 0.9180 I N10= 554° ~ 10.5° 1.80518 1.60311 1.80400 1.88300 1.51742 1.84666 1.58313 1.84666 1 .58313 1.51633 ν I = 25.4 ν 2 = 60.7, 5 = 52.4 ν 5 = 23.8 νIO = 12th aspherical surface Ro = 1.6027 C = 6. 9020XIO-' 17th Surface aspherical surface Ro = 2.1040 = 4 6502XlO-' B = Numerical example 3 R] = R2 = R3 = 13.1045 3, ], 293 -9.1395 15.9429 0.8170 1~57 FNO = 1 : 2.05 ~ 258 DI = 0.1385 D 2 = 0.5692 D 3 = 0.0308 D 6 = 0.0923 D 7 = 0.3674 2ω: 524° ~ 99°N 1 = 1.80518 N 2= 1.60311 1.77250 1.78590 ν 1= 25.4 ν 2=607 NIO≠ 1.58313 1.84666 1.58313 1.51633 ν10 12th aspherical surface Ro= 1 C=9.5912xlO− ' 17th aspherical surface RQ = -2,4869 B = -5 Knee 3 B: 3806 By adopting a lens configuration in which the fourth group is moved, the entire lens system is made smaller, achieving a zoom ratio of about 6 and good aberration correction over the entire zoom range, while minimizing aberration fluctuations during focusing. It is possible to achieve a rear focus zoom lens with an F number of about 1.8 to 2.0 and high optical performance.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of an embodiment showing the paraxial refractive power arrangement of the present invention, Fig. 2 is a cross-sectional view of a lens of Numerical Example 1 of the present invention, and Figs.
FIG. 5 is a diagram showing various aberrations of numerical examples 1 to 3 of the present invention. In the aberration diagrams, (A) is at the wide-angle end, (B) is at the middle, and (C) is at the wide-angle end.
is an aberration diagram at the telephoto end zoom position. , In Figures 1 and 2, r, n, ■, ■, sp are the first group, second group, third group, fourth group, aperture stop, and d is d.
line, g is the g-line, ΔM is the meridiocal image plane, and ΔS is the sagittal image plane. FNO/1.95 Spherical aberration? 8.0' Astigmatism w=28. o'W=Z8.0' Distortion (%) Lateral chromatic aberration (9 lines) FNO/Z, 6z Spherical aberration w= 5.3" Astigmatism 11.4' W=tt, lj' Distortion (%) Lateral chromatic aberration (9 lines) 5.3″ W=5.3′ Distortion (%) Lateral chromatic aberration (9 lines) FNO/ 7.85 Spherical aberration Spherical aberration W= , ? 7.7" Astigmatism Astigmatism W=27.7°w=z't, 7a Distortion (powder chromatic aberration of magnification (9 lines) w=ff, 3°W=IL3m Distortion (%) Lateral chromatic aberration (9 lines) ) W 5.26 @ w=, 5・26 Distortion (%) Lateral chromatic aberration (9-line distortion (%)) Lateral chromatic aberration (9-line FNo/2.43 Spherical aberration Spherical aberration W=/θ, da Astigmatism Astigmatism W = to・5゜10・5゜distortion (%) Lateral chromatic aberration (9 lines) W= 4.'14'' w=4, q4゜distortion (lateral chromatic aberration (9 lines)

Claims (1)

[Claims] (1) In order from the object side, the first group has a positive refractive power, the second group has a negative refractive power, the third group has a positive refractive power, and the fourth group has a positive refractive power. The second lens group is moved toward the image plane to perform zooming from the wide-angle end to the telephoto end, and the fourth lens group is moved to correct image field fluctuations that occur with zooming. Perform focusing and set the focal length of the i-th group to F_i, the focal length of the entire system at the telephoto end, the open F number, and the half angle of view to F_T, F_N_
When O_T and ωT, 0.02<F_1^2・F_N_O_T・tanωT/
A zoom lens characterized by satisfying the condition F_r^2<0.1. (2) The zoom lens satisfies the following conditions: 0.09<|F_2/F_T|<0.2 0.59<|F_3/F_4|<0.85 zoom lens.