JPH03215810A - Rear focus type zoom lens - Google Patents

Abstract

PURPOSE:To obtain the rear focus type zoom lens which has small aberration variation in focusing and high optical performance by setting the refracting power values of four lens groups and the movement conditions of the respective lens groups and a stop, and employing lens constitution wherein a 4th group is moved at the time of the focusing. CONSTITUTION:The stop SP is arranged between a 2nd group II and a 3rd group III. When the power is varied from the wide-angle end to the telephoto end, the 2nd group is moved monotonously to the image plane side and the stop SP, 3rd group III, and 4th group IV are moved to the object side indepen dently of one another while having convex tracks. A zoom type like this is employed to give the 1st group a 3.5 maximum effective diameter, which is much closer than a 2.98 minimum effective diameter determined by the F num ber at the telephoto end. Thus, while the lens effective diameter of the 1st group is reduced, a 56 deg. wide view angle is easily obtained at the wide-angle end, and the excellent optical performance is obtained over the entire power variation range.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a rear focus type zoom lens, particularly a zoom lens with a variable power ratio of 8 and an F number of 1.8, which is used in photographic cameras, video cameras, broadcast cameras, etc. The present invention relates to a rear focus type zoom lens having a relatively large aperture ratio and a high variable power ratio, and having four lens groups as a whole.

(Prior Art) Various types of zoom lenses for photographic cameras, video cameras, etc. have been proposed that employ the so-called rear focus method, in which focusing is performed by moving lens groups other than the first lens group on the object side. ing.

In general, rear focus type zoom lenses have a smaller effective diameter of the first group compared to zoom lenses that focus by moving the first group, making it easier to downsize the entire lens system, and it is also suitable for close-up shooting, especially at 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.

As such a rear focus type zoom lens, for example, Japanese Patent Laid-Open No. 82-24213 discloses, in order from the object side, a first group with positive refractive power, a second group with negative refractive power, and a third group with positive refractive power. In a zoom lens consisting of four lens groups, the fourth group having positive refractive power, the first and third groups are fixed, and the second group is moved in the direction to change the magnification. The surface variation is performed by moving the fourth group. and the fourth
Focusing is performed by moving the group.

Furthermore, in JP-A-58-]fi0913, 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 third group has a positive refractive power. It has four lens groups in four groups, and magnification is changed by moving the first and second groups.
The image plane changes due to zooming are performed by moving the fourth lens group.

Focusing is performed by moving one or more of these lens groups.

(Problems to be Solved by the Invention) In general, when a rear focus method is adopted in a zoom lens, the entire lens system becomes smaller as described above, and rapid focusing becomes possible, which further facilitates close-up photography. The following features can be obtained.

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.

For example, in the above-mentioned Japanese Patent Application Laid-Open No. 62-24213,
If you try to make the angle of view wider at the wide-angle end, the position of the off-axis light beam entering the first group will become higher in the intermediate zoom range, and as a result, the effective diameter of the lens of the first group will increase. .

Generally, a method for reducing the effective diameter of the first lens group is to shorten the distance between the first group and the diaphragm during zooming. At this time, when the aperture is moved with zooming, if the amount of movement of the aperture becomes large, the incident position of the light beam on the rear lens group, for example, the third group, will differ greatly for each zoom range. There is a problem in that the aberrations of the off-axis light beam increase, making it very difficult to correct the aberrations favorably as a whole.

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

The present invention adopts a rear focus system, and when achieving a large aperture ratio and high zoom ratio, it prevents the overall size of the lens system from increasing, and covers the entire zoom range from the wide-angle end to the telephoto end.
Another object of the present invention is to provide a rear focus type zoom lens that has good optical performance over a wide range of distances, from objects at infinity to objects at short distances.

(Means for Solving the Problems) The rear focus type zoom lens of the present invention includes, in order from the object side, a first group with a positive refractive power, a second group with a negative refractive power, an aperture, and a diaphragm with a positive refractive power. It has four lens groups: a third group and a fourth group with positive refractive power, and when changing the magnification from the wide-angle end to the telephoto end, the second group is moved to the image plane, and the aperture, The third group and the fourth group are moved independently of each other so that they have convex trajectories toward the object side, and when focusing, the fourth group
It is characterized by moving groups.

(Example) FIG. 1 is a schematic diagram of an example showing the paraxial refractive power arrangement of a rear focus type zoom lens according to the present invention. FIG. 2 is a sectional view of a lens according to a numerical example of the present invention, which will be described later.

In the figure, ■ is the first group with positive refractive power, II is the second group with negative refractive power, ■ is the third group with positive refractive power, and ■ is the fourth group with T refractive power.
It is a group. SP is an aperture diaphragm arranged between the second group n and the third group (2).

When changing the magnification from the wide-angle end to the telephoto end, the second group is moved monotonically toward the image plane as shown by the arrow, and the aperture sp, the third group, and the fourth group have convex trajectories toward the object side. are moved independently from each other.

In this example, by adopting such a zoom type, the maximum effective diameter of the first group is 3 in the numerical example described later.
．． 5, which is the minimum effective diameter determined by the F number at the telephoto end. This value is quite close to that of 98 (7.672.55).

In this way, in this example, while reducing the effective diameter of the first lens group, it is possible to easily widen the field of view to 56° at both wide ends, and to obtain good optical performance over the entire zoom range. ing.

In addition, 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 is fixed during zooming and focusing.

In this embodiment, the fourth group is moved to change the magnification, 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, the straight line 4c in the figure
This is done by rolling the fourth group forward as shown in the figure.

The aperture diaphragm SP is placed between the second and third groups, and when changing magnification, it is moved as described above to reduce aberration fluctuations caused by changing magnification, and to reduce the distance between the lens groups in front of the aperture diaphragm. By making it shorter, the effective diameter of the first lens group can be easily reduced.

In the present invention, in order to downsize the entire lens system and correct various aberrations in a well-balanced manner over the entire zooming range while ensuring a predetermined zoom ratio, the distance between the principal points of the second and third groups is set to e.
23. The distance between the aperture and the front principal point of the third group is LS
3, it is preferable to satisfy the condition 0.15<LS3/e23<0.7=.(1) over the entire magnification range.

If the lower limit of conditional expression (1) is exceeded, the distance between the diaphragm and the first group becomes too long, which is not good because the effective diameter of the lens of the first group increases. If the upper limit is exceeded, the distance between the diaphragm and the third group becomes too long, and as a result, the height of incidence of off-axis light beams on the third and fourth groups becomes too high, making it difficult to properly correct off-axis aberrations. It's getting difficult.

In addition, in the present invention, in order to particularly correct off-axis aberrations well over the entire zooming range, the distance between the principal points of the third group and the fourth group should be e34, and the distance between the principal points of the third group and the fourth group at the wide-angle end should be set to e34. When the distance between the principal points is ew 3 4, it is preferable to satisfy the condition 0.5<e34/ew34<1.5 - (2) over the entire magnification range.

In either case, if the lower limit or upper limit of conditional expression (2) is exceeded, the height of the incident position of the off-axis light beam into the fourth group will change greatly as the magnification changes, and aberrations due to the off-axis light beam will occur as the magnification changes. As the fluctuations become larger, it becomes difficult to satisfactorily correct these off-axis aberrations.

In addition, in the present invention, in order to correct various aberrations in a well-balanced manner over the entire screen, in order from the object side, the third group is a positive lens with both lens surfaces convex, a meniscus negative lens with a convex surface facing the image side, The fourth group consists of a meniscus-shaped positive lens with a convex surface facing the object side, a meniscus-shaped negative lens with a convex surface facing the object side, a positive lens with both lens surfaces convex, and a lens with both lens surfaces convex. It is better to use a positive lens.

Next, numerical examples of the present invention will be shown. In numerical examples R
j 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 vi are the refractive index and Abpe number of the glass of the i-th lens, respectively, in order from the object side.

Note that R24 and R25 are parallel plane plates such as a face plate and a filter. Table 1 also shows the distance between the principal points of the i-th group and the (i+1)-th group as ei (i+1).

Numerical Example F=]. 0 ~ 7.6 R1 = R 2 = R3 = R4 - R5 = R6 = R7 = R8 = R9 = R10 = R11 = R12 = R13 = R14 - R15 - R16 - R17 = R18 = R ] 9 - 12.09 + 4 .453 33.004 4.364 13.348 539.302 1.124 -3.896 1.27+ 6.603 Aperture 8.970 2.892 -2.181 3.642 2.163 3.39] 9. 395 2.352 FNO=] : 1.85 ~ 2.55 2ω-27.9' ~ 40 DI= 0.195I N D 2= 0.6946 N D 3= 0.0325 D 4= 0.4774 N D5-Variable D 6 = 0.1301 N D 7 = 0.42] ID 8 = 0.1301 N D 9- 0.4133 N D10 = Variable old 1 = Variable DI2 = 0.5815 N D13- 0.0435 014- 0.1301 N 8=]. 8051
8DI5- 0.0244 D]6=0.3318 N D17=Variable D]8=0.1301 D]9=0.1282 ]=] . 80518 5-1.5]633 7=1. 69680 3=1. 69680 9=1.51633 2=] . 69680 6=1.841i86 4=I. 83400 N10=1. 84686 1=25.4 2=55.5 3=55.5 4-37.2 5-64.1 6-23.9 7=55.5 8-25.4 9=64. ] υ10−23.9 11 R20= 9.284 R21− −3.094 R22− 2.122 R23−103.382 824− ω R25= (1) D20= D21−D22 Wealth D23= D24−0.532] 0.0244 0.4809 0.8130 0.6992 Nl]=]. 60311 Nl2=] . 51833 N+3=]. 51633 ν]]=60.7 v 12=64.1 v 13J4. ] f, -5.85 f3 = 3.47 f2 f, = -1. 33 3.52 1 2 (Effects of the Invention) According to the present invention, as described above, the refractive power of the four lens groups and the movement conditions of each lens group and aperture during zooming are set, and the fourth group is moved during focusing. By adopting a moving lens configuration, the entire lens system can be miniaturized while achieving good aberration correction over the entire zoom range with a variable power ratio of around 8, and high optical performance with little aberration fluctuation during focusing. It is possible to achieve a rear focus type zoom lens with a wide angle of view of 56° at the wide-angle end, and a large aperture ratio with an F number of 1.8.

[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 according to a numerical embodiment of the present invention, and FIG.
The figures are various aberration diagrams of numerical examples 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, I, II, m, IV are 15 in order, 1st, 2nd, 3rd, 4th group, d is d line, g is g line, ΔM
is a meridian image plane, ΔS is a sagittal image plane, and sp is an aperture stop. 1 6 Fig. 3 (A') Spherical aberration Astigmatism IflO convergence Astigmatism 3 Fig. (B') Distortion IIil fraud No. 70 Fig. (C) Distortion convexity Yield (''A) Magnification color scale

Claims (1)

[Claims] (1) From the object side, the first group has a positive refractive power, the second group has a negative refractive power, the diaphragm, the third group has a positive refractive power, and the fourth group has a positive refractive power. It has four lens groups, and when changing the magnification from the wide-angle end to the telephoto end, the second group is moved toward the image plane, and the aperture, third, and fourth groups are all moved. A rear focus type zoom lens characterized in that the fourth group is moved independently of each other so as to have a convex locus on the object side, and the fourth group is moved when focusing. (2) The distance between the principal points of the second group and the third group is e23, and the distance between the aperture and the front principal point of the third group is L.
2. The rear focus type zoom lens according to claim 1, wherein the rear focus type zoom lens satisfies the following condition: 0.15<LS3/e23<0.7 over the entire zoom range when S3 is set. (3) When the distance between the principal points of the third group and the fourth group is e34, and the distance between the principal points of the third group and the fourth group at the wide-angle end is ew34, 0.5<0.5 over the entire magnification range The rear focus type zoom lens according to claim 1 or 2, characterized in that it satisfies the following condition: e34/ew34<1.5.