JPH0387803A - Real image type variable power finder - Google Patents

Abstract

PURPOSE:To realize the high-power, small-sized real image type variable power finder by moving a 2nd lens group toward the ocular side and varying the power from the wide-angle side to the telephoto side, and correcting variation in finder diopter by the movement of a 3rd lens group and satisfying specific conditions. CONSTITUTION:A positive objective lens consists of a 1st positive lens group 11, the 2nd negative lens group 12, the 3rd positive lens group 13, and a 4th positive lens group 14 in order from the object side on the whole. Then a positive secondary image forming lens 15, a field lens 16, and an ocular 17 are arranged behind them. When the power is varied from the wide-angle end to the telephoto end, the 2nd lens group 12 is moved toward the ocular side and the resulting variation in finder diopter is corrected by the movement of the 3rd lens group 13. Further, inequalities hold, where FW is the focal length of an objective lens part at the wide-angle end, FT is the focal length at the telephoto end, and fi is the focal length of each lens group.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a variable magnification finder, particularly a variable magnification finder with a high magnification and high magnification suitable for photographic cameras, video cameras, electronic still cameras, etc. .

[Prior art]

Conventionally, in cameras where the photographing system and finder system are configured separately, when the photographing system is a variable magnification system, it is desirable to change the field of view magnification of the finder as the photographing system changes the magnification. It is preferable that the finder system has a structure that allows a desired variable power ratio to be easily obtained.

For example, in Japanese Patent Application Laid-Open No. 53-63014, in an inverted Galilean finder consisting of, in order from the object side, an objective lens with negative refractive power and an eyepiece lens with positive refractive power,
We have proposed a variable magnification finder in which the objective lens is divided into two lens groups with positive and negative refractive powers, and of these, the lens group with negative refractive power is moved to the eyepiece side to perform magnification change. However, in general, reverse Galilean finders are
Since the structure is such that a virtual viewfinder image formed by an objective lens is observed, there are drawbacks such as the viewfinder field of view changing depending on the position of the eye and the outline of the viewfinder field becoming unclear.

On the other hand, a real image finder has a structure in which a real finder image formed by an objective lens with positive refractive power is observed, so a field mask can be provided at the real image position, and the outline of the finder field of view is It has the advantage of being clear, and the viewfinder field of view does not change even if the position of the eye changes.

By the way, as a technique for changing the field of view magnification of a real image finder, for example, Japanese Patent Application Laid-Open No. 1986-
It has been proposed in Publication No. 156018 and the like.

FIG. 2 shows a sectional view of the lens. The second lens group 23 in the objective lens group is moved toward the object side to change the magnification, and the accompanying change in finder diopter is measured by the first lens group 23.
This is corrected by moving the . In the figure, 21 is a protective glass, 22 is a first lens group, and 23 is a second lens group.
The lens group 24 is a third lens group, and 22, 23, and 24 constitute an objective lens group. 15 is a secondary imaging lens;
16 is a field lens, 17 is an eyepiece lens, 18 is an eye point, 19 is a primary imaging surface, and 20 is a secondary imaging surface.

[Problem that the invention seeks to solve]

However, the magnification ratio of the finder disclosed in the above-mentioned publication is only about 2 times.

In recent years, there has been a growing demand for relatively compact cameras to be equipped with zoom lenses with a high variable magnification ratio, but the above-mentioned known variable magnification finder can achieve a relatively small size. Having a sufficient magnification ratio of
It was difficult.

The purpose of the present invention is to maintain a relatively high variable power ratio while maintaining a small size.
The object of the present invention is to provide a real image variable magnification finder with good optical performance.

[Configuration to solve the problem]

Consists of at least three lens groups, in order from the object side, a first lens group with positive refractive power, a second lens group with negative refractive power, and a third lens group with positive refractive power, and has positive refractive power as a whole. In a variable magnification finder that includes an objective lens section and an eyepiece section having positive refractive power and arranged behind the objective lens section, by moving the second lens group toward the eyepiece side, The magnification is changed toward the telephoto side, and the accompanying change in the finder diopter is corrected by moving the third lens group, and the focal length at the wide-angle end of the objective lens section is changed to the focal length at the FW telephoto end. F7 The focal length of the first lens group is f's The focal length of the second lens group is fff The focal length of the eyepiece is f The lens surface on the object side of the lens placed closest to the object side in the third lens group When the radius of curvature of the lens surface on the eyepiece side of RA is R8, 1< f r / F a < 2.5 (1
)t<l fa/Fw |<2.5...(2)
However, fn<O o< l R,/RA1 <0.5...(3)
0, 2<Fw/f, <1...(
4) The purpose is to satisfy the following conditional expression.

〔Example〕

FIG. 1 is a schematic diagram showing an embodiment of a real image variable magnification finder according to the present invention. In the same figure, 11.12,1
3,14. are the first lens group with positive refractive power, the second lens group with negative refractive power, the third lens group with positive refractive power, and the fourth lens group with positive refractive power, and 11, 12, 13 .1
4 constitutes an objective lens with positive refractive power as a whole. 15 is a secondary imaging lens, 16 is a field lens, 17 is an eyepiece 18 is an eye point, 19 is a primary imaging surface, and 20 is a secondary imaging surface. When changing the magnification from the wide-angle end to the telephoto end, the second lens group 12 in the objective lens moves toward the eyepiece side, and the third lens group 13 draws a convex locus toward the object side, thereby changing the diopter. The magnification is changed without . Here, the first and fourth lens groups 11 and 14 are fixed during zooming, and in particular, the first lens group 11 is fixed during zooming to facilitate a sealed structure, and if a protective glass is placed in front. Even without the first lens group 11 and the second lens group
This prevents dust and the like from entering between the lens group 12 and the lens group 12. The 41st non-lens group 14 is a so-called field lens that is arranged near the synthetic focal plane of the first to third lens groups and condenses the light beams that have passed through the first to third lens groups in the optical axis direction so as not to diverge. It is equipped with the following functions.

With the above-described configuration, the light beam from the subject passes through the first to fourth lens groups and then forms an image on the primary imaging plane 19 with the upper, lower, right, and left sides reversed. Next, a secondary imaging lens 15 and a field lens 16 placed behind the primary imaging plane 19 convert the subject image, which has been vertically and horizontally inverted on the primary imaging plane, onto the secondary imaging plane 20 into a regular image. The object image on the secondary imaging plane 20 is observed at the eyepoint 18 through the eyepiece 17.

In addition, when using an image sensor with a relatively small shooting screen size, such as a still video camera that has been rapidly developing in recent years, it is possible to achieve high zoom magnification and even shoot at close range without making the shooting lens relatively large. It becomes easier to do so. As a result, unlike the conventional method, a problem arises in that the diopter of the external viewfinder is not sufficiently adjusted, especially when photographing a close-up object during telephoto shooting. In this embodiment, when performing such close-range photography, the first lens group of the objective lens system is extended in the optical axis direction as a focus lens, thereby easily correcting the diopter deviation for the close-range subject. .

Next, in order to obtain a finder image with high zoom ratio and good optical performance in the real image finder of the present invention, the focal length at the wide-angle end of the objective lens section must be set to F'II + the focal length at the telephoto end to FTs. The focal length of the first lens group is fIs The focal length of the second lens group is fax The focal length of the eyepiece is fl
, When the radius of curvature of the lens surface on the object side of the lens disposed closest to the object side in the third lens group is RA1, and the radius of curvature of the lens surface on the eyepiece side is RB, 1<fr/F7<2.5 ・(1) 1<
l f */F-1<2.5...(2) However
fa<0 0<JR, /RA I<0.5...(3) o,
2×IFW/f, |<1 (4) It is preferable to satisfy the above-mentioned condition. The technical meaning of the above conditional expression will be explained below. Conditional expression (1) is a condition regarding the focal length of the first lens group, and if it exceeds the upper limit, it becomes difficult to obtain a sufficient variable power ratio and the overall length of the finder becomes long, which is not preferable. On the other hand, if the lower limit is exceeded, the curvature of field will be overcorrected and spherical aberration will be undercorrected at the telephoto side, which is not preferable.

Also, this conditional expression (1) relates to the case where the first lens group is moved as described above in order to correct the diopter change due to the object distance of the finder, and if the upper limit of the conditional expression is exceeded, the diopter change will be corrected. The amount of movement of the first lens group for correction becomes large, which increases the overall length of the finder and the diameter of the first lens, which is undesirable. Furthermore, conditional expression (2) relates to compactness of the finder system, and if the upper limit of the conditional expression is exceeded, the amount of movement of the second group due to zooming becomes large (
I don't like it. If the lower limit is exceeded, it is advantageous for compactness, but distortion aberration will be insufficiently corrected on the wide-angle side, which is undesirable.

Next, conditional expression (3) is mainly related to the correction of coma aberration; if the lower limit of the conditional expression is exceeded, halo generation due to the upper light beam will increase at the telephoto side, and if the upper limit is exceeded, the halo will be lower at the wide-angle side. This is undesirable because it causes comatic aberration.

Generally, in a finder system, if coma aberration remains, it will appear as image fluctuation when the observer shakes his or her eyes, so in this example, by specifying the lens shape as described above, Comatic aberration is well corrected, and even if you shake your eyes a little, you can still get a good viewfinder image with no fluctuations.

Conditional expression (4) is a conditional expression regarding the ratio of the focal lengths of the objective lens and the eyepiece lens. If the upper limit of the conditional expression is exceeded,
This is not preferable because the eye point cannot be made long and various aberrations, especially curvature of field, are significantly worsened. If the lower limit of the conditional expression is exceeded, the eye point can be set longer, but the overall length of the finder becomes longer, which is not preferable.

- In the old invention, despite the simple configuration, in order to reduce aberration fluctuations during zooming and achieve good aberration correction over the entire zooming range, the first lens group in the objective lens group must be When the second lens group is composed of two negative lenses and a particularly high zoom ratio is desired, the object side lens of the second lens is placed on the eyepiece side, and a concave negative lens is placed on the eyepiece side. The side lens consists of two negative lenses with a strongly concave surface facing the object side, the third lens group consists of at least two positive lenses, and the object side lens of the eyepiece has a strongly convex surface facing the eyepiece. It is preferable that the eyepiece side lens be composed of two biconvex positive lenses, and the fourth lens group be composed of one positive lens with a strongly convex surface facing the object side. Further, in the present invention, lens cross-sectional views of examples 1 to 3 in which one of the surfaces of the first lens group is aspheric are shown in order to correct distortion aberration in particular. In numerical examples,
Ri is the radius of curvature of the i-th optical element in the order of progression of the light flux,
Di is the i-th lens thickness and air spacing, and Ni and Vi are the refractive index and Abbe number of the i-th optical element.

Further, the mark * in the examples represents an aspherical surface, and the aspherical surface coefficient is the paraxial radius of curvature Ra of the shape of the aspherical surface.

When the traveling direction of the light beam on the optical axis is the X-axis, and the direction perpendicular to the X-axis is the Y-axis, these are shown as B, C, D, etc.

Further, Table 1 shows the relationship between each of the above-mentioned conditional expressions and various numerical values in the numerical examples.

Numerical Example 1 23 *mark is aspherical surface E=5°798X10-IO E=1°943X10-' Numerical Example 2 *marked is aspherical surface" 8°26X10 Numerical Example 3 Ri Ri 11°71 -48. 94 ν l *mark is aspherical surface C = 3°D = -7°E = 6°12 Xl0-'604X10-' 28X10 Numerical Example 4 Ri 1 -144. 93-20. 709 1.8 6 1° Wide-angle end Intermediate Telephoto end D 2 1.383 5.842 14.
123D 6 14.24 8.606
1.643r) 10 12.377 13.55
1 12.233 * mark is an aspherical surface C = 1°D = -4°858X10 @ 437xlO-” [Effects of the Invention] As explained above, according to the present invention, when changing the magnification from the wide-angle end to the telephoto end, , 1. The fourth lens group is fixed, the second lens group is moved to the eyepiece side, and the third lens group is moved to correct for finder diopter change, and each lens is defined by shape and conditional expression. By this, the magnification ratio is 3.
Despite the high zoom ratio of ~3.5x, aberrations can be corrected well over the entire zoom range.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an embodiment of the present invention. FIG. 2 is an explanatory diagram of the prior art. 3 to 6 are cross-sectional views of numerical examples 1 to 3 of the present invention, respectively. 7 to 10 are aberration diagrams of numerical examples 1 to 3 of the present invention, respectively. In the figure, 11 is the first lens group, 12 is the second lens group, 13 is the third lens group, 14 is the fourth lens group, 16 is the field lens, 20 is the secondary imaging plane, 17 is the eyepiece, and 18 is the The pupil, AM, is a meridional image plane, and AS is a sagittal image. (Zi off pig) (te? old?) (〃) (child 1 year old) 9-) (1000 oz.) ('/, ) (diozuda) (watch C) (''/,) (te'it 勺-) (punishment opter) (610n(tej opter 2) (te 1 of goon(D/-) ('/-) -IJWI/ tongue missing FIEJqnu [L!σσ-!00
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Claims (4)

[Claims] (1) Consisting of at least three lens groups in order from the object side: the first lens group with positive refractive power, the second lens group with negative refractive power, and the third lens group with positive refractive power, and has positive refraction as a whole. In a variable magnification finder composed of an objective lens section having a refractive power and an eyepiece section having a positive refractive power disposed behind the objective lens section, by moving the second lens group toward the eyepiece side. Changing the magnification from the wide-angle side to the telephoto side, and correcting the change in finder diopter caused by this by moving the third lens group,
The focal length at the wide-angle end of the objective lens section is F_W, the focal length at the telephoto end is F_T, the focal length of the first lens group is f_I,
The focal length of the second lens group is f_II The focal length of the eyepiece is f_e The radius of curvature of the lens surface on the object side of the lens placed closest to the object side in the third lens group is R_A The curvature of the lens surface on the eyepiece side When the radius is R_B, 1<f_ I /F_T<2.5...(1) 1<|f_II/F_W<2.5...(2) However, f_I
I<0 0<|R_B/R_A|<0.5...(3)0.2<
A real image variable magnification finder characterized by satisfying the following condition: F_W/f_e<1 (4). (2) A real-image variable magnification finder according to claim 1, wherein the first lens group is extended in the optical axis direction to correct a diopter deviation of the finder with respect to a close object. (3) The first lens group is composed of one positive lens, the second lens group is composed of at least one negative lens, and the third lens group is composed of two positive lenses. The real image variable magnification finder described in item 1. (4) A real image variable magnification finder according to claim 1, wherein in the objective lens section, one of the surfaces of the first lens group is an aspherical surface.