JPH04179908A - Small-sized real image type variable power finder - Google Patents

Abstract

PURPOSE:To shorten a movable part and to improve visibility by forming this finder of an objective lens part which varies power by moving 2nd and 3rd lens groups and an ocular part and making the focal distances of 1st and 2nd lens groups of the objective lens part satisfy a specified conditional expression. CONSTITUTION:The 1st to the 3rd lens groups are provided with negative, positive, and negative lens groups in order from an object side. The object lens part which varies the power by moving the 2nd and the 3rd lens groups and the ocular part are formed, and the focal distances of the 1st and the 2nd lens groups of the objective lens, is made to satisfy the conditional expression of 1<¦f,¦/f2<1.2, supposing that the focal distances of the 1st and the 2nd lens groups are f1 and f2, respectively. The power is varied by principally moving the 2nd lens group and the movement of an image surface with the variable power is corrected by moving the 3rd lens group. Since the movement of the 3rd lens group hardly contributes to the power varying action, the 3rd lens group only has to have weak power by which the movement of the image surface is corrected. Thus, the entire length obtained at the time of assembling the finder in a camera is shortened and the visibility is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a small real-image type variable magnification finder having a short overall length of a variable magnification section and suitable for a lens shutter camera.

(Prior art) Conventionally, virtual image finders have been commonly used as viewfinders for lens-shutter cameras due to their cost advantages, and even in the case of cameras equipped with zoom lenses, virtual image variable magnification was initially used. A finder was used.

However, in virtual image finders, the entrance pupil is located at the rear, so the diameter of the front lens inevitably becomes large, and the brightness of the field of view and the sharpness of the field frame are inferior to real image finders. By the way, real image finders require an objective lens and an eyepiece, so even if an erect prism is used,
The overall length of the finder (the distance from the frontmost surface of the objective lens to the rearmost surface of the eyepiece lens) is quite large. In the past, the body size of zoom cameras was much larger than that of single focus cameras, so the length of the viewfinder was not a particular issue. However, as fixed-focus cameras become thinner and zoom cameras are also required to be thinner, the length of the viewfinder becomes a factor that prevents cameras from becoming thinner.

The overall length of the finder can be shortened by bending the optical axis with a reflective surface, so in order to shorten the finder length, the length in front of the first reflective surface can be shortened.

Since the reflective surface is usually placed behind the movable part, this corresponds to shortening the distance from the first surface of the objective lens to the movable part.

In Japanese Patent Application Laid-Open No. 1-309020, a two-group negative and positive zoom lens is used as an objective lens to obtain a finder with a short movable part. However, with such a two-group configuration, distortion at the wide-angle end and the telephoto end cannot be completely corrected, and barrel-shaped and pincushion-shaped aberrations become noticeable, respectively. Furthermore, if the first lens group is moved, dust tends to enter the finder due to its structure, which is undesirable.

(Problems to be Solved by the Invention) The present invention has a short overall length when incorporated into a camera, and provides good visibility.
The object of the present invention is to obtain a real-image variable magnification finder suitable for thin cameras that does not cause the above-mentioned problems.

(Means for solving the problem) In order to achieve the above object, in the present invention, a real image variable magnification finder includes, in order from the object side, a negative first lens group, a positive second lens group,
an objective lens unit having a negative third lens group and performing variable magnification by moving the second lens group and the third lens group;
When the focal lengths of the 1.2nd lens group of the objective lens section are fl and f, respectively, the following conditions are satisfied: 1<If, l/f, <1.2.

(Function) In the finder of the present invention, the magnification is changed mainly by moving the second lens group, and the movement of the image plane accompanying the magnification is corrected by moving the third lens group. Since most of the movement of the third lens group does not contribute to the variable magnification effect, it is sufficient that the third lens group has a weak power sufficient to correct the movement of the image plane. Since an erecting prism is disposed between the movable part and the field frame, the first group is set as a negative group to ensure back focus.

The conditional expression relates to the focal lengths of the first and second groups, and as f exceeds the lower limit and increases, the amount of movement of the second group increases, and the movable portion becomes longer.

If f2 exceeds the upper limit and decreases by /J), the spherical aberration increases and the error sensitivity increases, which is not desirable.

Further, in the present invention, in order to correct strong negative distortion, it is desirable to use aspheric surfaces whose curvature becomes gentle off-axis in the first and second groups. Furthermore, if an aspheric surface whose curvature becomes stronger off-axis is used in the negative third lens group, distortion and spherical aberration can be corrected even more effectively.

(Example) Examples of the variable magnification finder of the present invention will be shown below. In this embodiment, the second and third groups each consist of one lens, and each of these two lenses has an aspherical surface on its object side surface.

The symbols in the table indicate the following.

r: Paraxial radius of curvature d: On-axis spacing nd: Refractive index In the near-Sobet number table, * indicates an aspheric surface, and its shape is orthogonal with the origin at the apex of the surface and the optical axis direction as the X axis. In the coordinate system, the apex curvature is expressed as h=F7-T77, where C1 is the conical coefficient and A4 and A are the aspherical coefficients.

[Example 1] Surface number r d 14 N'd
1 9.760 1.50 1.492 57.
02 pieces 3.987 4.42 3 20.078 1,50 1.492 57.
04 books 15.227d.

5 pieces 18.944 2.50 1.492 57.
06 -8.059 7 ton -11,7721,501,4925
7,08 -16.700 d.

9 28.370 25.20 1,492
57.010 -26.400 1.0
011 00 -23.50 1.
492 57.012 -23.200 1
．． 0013 26.358 2.00 1
．． 583 30.014, 13.436
2.5015 17.059 3.50
1.492 57.016 -23.548
18 17 (pupil) Wide-angle end Intermediate Telephoto end d410, 58 6.03 '2.67d,
0,50 2,17 8.41d, 0.
50 3.38 0.50 fine view magnification 0
．． 40~0.77 maximum angle of incidence ω=28°~1
4゜1' f, I /f, = 1.09 Aspheric coefficient 2nd surface 4th surface = -0,20023 = -3,9399A4 =
0.85099XIO-' A4=-0,208
91XIO-'A, = 0. OA, = 0.0 5th side 7th side = -0,72123 = 1.7000A, =
-0,41757XIO-' A4= -0,115
05xlO-'A, = -0,18660XlO-'
A, = 0.0 [Example 2] Surface number r d n, 1 νd
1 10.597 1.50 1,492 57.
02 pieces 4,1364.40 3 -40.394 1.50 1.492 57
．． 04 -324.90 d4 5 pieces 17.7172.501.49257.06
-8.1750 d.

7 pieces 14.032 1', 50
1.492 57.08 ~20.750
d.

9 32.612 24,40 1.492 57
．． 010 -26.400 1.0011
(X) 26.00 1,492
57.012 -25.000 0.5
013 30.437 1.50 1.4
92 57.014 13.133 1
．． 3015 17.007 3.20 1
,492 57.016 lines - 21,86718 17 (pupil) ω Wide-angle end Intermediate Telephoto end d4to, 10 5.9 (12,50d, 0
,50 1,90 8.10d, 1,00 3.
801.00 Finder magnification 0.41~0.77 Maximum angle of incidence
ω=28°~14°l f, I/f,=1.
06 Aspherical coefficient 2nd surface 5th surface=-0,23630=-0,83776A4=
0.15950xlO-” A4=-0,508
81XIO-'A, = -0,10674xlO-'
A, = -0,18572xlO-'7th surface
To the 16th side = 2.8042 To = -0,3986
5A, = −0,14790xlO−” A4= −
0,55657xlO-'A, = 0.15444X
10-" A, = -0,1789]Xl0-" [Example 3] Surface number r d n, 1 v4
1 9.6670 1.50 1.583 30
．． 02 * 4.2671 3.933 -
32.28.8. 1.50 1.492 57.04
-73.404 d4 5 pieces 18.372 2.50 1
．． 492 57.06 -8.2515 d
.

7m -15,3881,50' 1.5
83 30.08 -22.096 d.

9 30.000 24.40 1.492 57
．． 010 -26.400 1.00 11 00 26.00 1.492 57.
012 -25.000 0.50 13 30.437 1.50 1.492 57
．． 014 13.133 1.3015
17.007 3.20 1.492
57.016 lines - 21,86718 17 (11ri 00 Wide-angle end Intermediate Telephoto end d, 10.57 6.28 2.82d, 0
．． 50 1.93 8.25d, 1.00 3.
86 1.00 fine index magnification 0.41~0°77 Maximum entry angle ω=28°~14°+f,
l /f, = 1.07 Aspheric coefficient 2nd surface 5th surface = -0,25395 = -0,69716A4 =
0.25242X10-”A,=-0,31
932X10-'A, = -0,33005xlO-"
A, = -0,20442xlO-'7th surface
To the 16th side = 3.4920 To = -0,3986
5A4= −0,11706xlO−” A4= −
0,55657xlO-'A, = 0.14667x
lo-"A, = -0,17891xlo-"(
Effects of the Invention) As described above, the real image variable magnification finder of the present invention has a short movable part for variable magnification, and although it is easy to configure it compactly to match the thickness of the camera,
As shown in the aberration diagram, various aberrations and their fluctuations are well corrected,
You can get a viewfinder with good visibility.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of one embodiment of the finder of the present invention, with the erecting prism shown unfolded. Section 2.3.
4 are aberration diagrams of Examples 1, 2, and 3, respectively.

Claims (1)

[Claims] In order from the object side, there is a negative first lens group, a positive second lens group, and a negative third lens group, and when the second lens group and the third lens group move, It consists of an objective lens section that performs magnification change, and an eyepiece section.
A real image variable magnification finder characterized by satisfying 1<|f_1|/f_2<1.2, where the focal lengths of two lens groups are f_1 and f_2, respectively.