JP2012168300A - Finder optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small sized finder optical system with great observation magnification.SOLUTION: The finder optical system for observing an image formed by an object lens by an eye piece lens through an optical system for erect image formation has the eye piece lens including: a first lens group with one piece of lens with negative refractive power; a second lens group with one piece of lens with positive refractive power for carrying out diopter correction by being moved in the direction of an optical axis; and a third lens group of positive refractive power in this order from the object side. In this case, the third lens group is composed of a lens of positive refractive power and a negative meniscus lens with a concave surface facing the pupil side in this order from the object side and the prescribed conditional expression is satisfied.

Description

  The present invention relates to a finder optical system used in an imaging apparatus such as a single-lens reflex camera.

  Conventionally, in a viewfinder optical system such as a single-lens reflex camera, a lens having a three-group three-lens configuration or a three-group four-unit configuration is known. In addition, there have been proposed ones intended to increase the viewfinder magnification and those intended to take a sufficiently long eye relief distance.

JP 2009-271385 A

Japanese Patent No. 4212295

  In general, in order to increase the observation magnification of the finder optical system, it is necessary to shorten the focal length of the eyepiece. In addition, in a finder optical system of a single-lens reflex camera, it is generally necessary to set the diopter of the eyepiece to −1 diopter. Therefore, from the focusing screen on which the subject image is formed to the main point position of the eyepiece. The actual focal length of the eyepiece is determined by the optical path length.

  Therefore, in order to increase the observation magnification of the finder optical system, the distance from the focusing screen to the principal point position of the eyepiece may be shortened. For this purpose, the optical path length of the pentaprism is shortened, or the finder optical system is May be arranged close to the pentaprism.

  However, with such a configuration, the viewfinder exit surface is recessed with respect to the back of the camera, making it difficult to bring the eyes close to the lens, and it is difficult to observe the entire field of view when trying to check the viewfinder image. End up.

  Downsizing the pentaprism is contrary to securing a high field of view. Furthermore, it has become necessary to incorporate an optical system for display in the finder and an optical system for photometry in the finder optical path, and the pentaprism tends to be enlarged.

  Therefore, in order to shorten the optical path length of the focusing screen and the eyepiece, a finder optical system capable of obtaining a larger observation magnification by arranging the principal point position of the eyepiece on the focusing screen side is disclosed in Patent Document 1. It is disclosed.

  On the other hand, in order to make the eye relief sufficiently long, it is generally necessary to enlarge the pentaprism and reduce the vignetting of the light beam by the pentaprism as much as possible. However, when the pentaprism is made larger, the optical path length from the focusing screen to the eyepiece becomes longer, and the observation magnification of the finder optical system becomes smaller.

  In this way, in the finder optical system of a single-lens reflex camera, it is contrary to increasing the eye relief distance sufficiently while increasing the observation magnification of the finder optical system.

  The eye relief means the distance from the final lens of the finder optical system to the eye point, but what is more important for the observer is often the distance from the back of the single lens reflex camera to the eye point.

  The larger the value from the back of the camera to the eye point, the better the finder than the eye relief value. In order to increase this value, it is necessary to increase the distance from the exit surface of the pentaprism to the eyepoint in viewfinder optics.

  By the way, in general, in a finder optical system of a single-lens reflex camera, there is a great demand for mounting a diopter correction mechanism that makes it possible to change the diopter according to the observer. Japanese Patent Application Laid-Open No. 2003-228867 discloses a finder optical system that is equipped with a diopter correction mechanism and that includes a plurality of eyepiece lenses and that allows a diopter correction by moving some of these lenses in the optical axis direction. It is disclosed.

  An object of the present invention is to solve the above-mentioned problems, to provide a good finder optical system equipped with a diopter correction mechanism, which reduces the processing cost, has a large observation magnification, has a long distance from the pentaprism exit surface to the eye point, and is equipped with a diopter correction mechanism. And

To achieve the above object, the finder optical system embodying the present invention is configured to observe an image formed by an objective lens with an eyepiece through an optical system for erecting an image. In order from the first lens group consisting of a single lens having a negative refractive power, a second lens group consisting of a single lens having a positive refractive power that performs diopter correction by moving in the optical axis direction, a positive lens The third lens group is composed of a lens having a positive refractive power and a negative meniscus lens having a concave surface facing the pupil side in order from the object side, and satisfies the following conditional expression: Features.
(1) -1.1 <f1 / f <-0.6
(2) -4.0 <f3n / f <-1.2
However, the focal length of the first lens group is f1, the focal length of the negative meniscus lens of the third group is f3n, and the focal length of the entire finder optical system when the diopter is -1 pt is f.

Furthermore, the finder optical system for carrying out the present invention is characterized in that, in the above invention, the object side surface of the positive lens in the third lens group is an aspherical surface, and satisfies the following conditional expression.
(3) -9.0 <Hmax / sag3p <-5.0
However, the maximum ray height on the aspheric surface side of the positive lens of the third lens group is Hmax, and the deviation in the optical axis direction between the paraxial spherical surface and the aspheric surface at the maximum ray height Hmax on the aspheric surface side of the positive lens is sag3p. .

Furthermore, the finder optical system embodying the present invention is characterized in that, in the above invention, the object side surface of the negative meniscus lens of the third lens group is an aspherical surface, and satisfies the following conditional expression.
(4) 8.5 <Hmax / sag3n <12.0
However, the maximum ray height on the aspherical side of the negative meniscus lens of the third lens group is Hmax, and the deviation in the optical axis direction between the paraxial spherical surface and the aspherical surface at the maximum ray height Hmax on the aspherical side of the negative meniscus lens is sag3n. And

  Furthermore, the finder optical system for carrying out the present invention is characterized in that, in the above invention, each lens constituting the eyepiece is formed of a plastic material.

  According to the finder optical system embodying the present invention, an object of the present invention is to provide an inexpensive finder that has a large observation magnification and can take a sufficiently long distance from the focusing screen to the observation optical system.

It is a lens structure which concerns on Example 1 of the finder optical system of this invention. FIG. 5 is an aberration diagram for Example 1 when the diopter is −1 dpt. FIG. 6 is an aberration diagram when the diopter is −3 dpt in Example 1. FIG. 6 is an aberration diagram when the diopter is +1.5 dpt in Example 1. It is a lens structure which concerns on Example 2 of the finder optical system of this invention. FIG. 6 is an aberration diagram when the diopter is −1 dpt in Example 2. In Example 2, it is an aberration figure when the diopter is -3dpt. FIG. 6 is an aberration diagram when the diopter is +1.5 dpt in Example 2. It is a lens structure which concerns on Example 3 of the finder optical system of this invention. FIG. 10 is an aberration diagram for Example 3 when the diopter is −1 dpt. FIG. 6 is an aberration diagram for Example 3 when the diopter is −3 dpt. FIG. 10 is an aberration diagram for Example 3 when the diopter is +1.5 dpt. It is a lens structure which concerns on Example 4 of the finder optical system of this invention. In Example 4, it is an aberrational diagram when a diopter is -1dpt. In Example 4, it is an aberrational diagram when a diopter is -3dpt. FIG. 10 is an aberration diagram for Example 4 when the diopter is +1.5 dpt.

  The finder optical system embodying the present invention includes an SI plate 1 for guiding superimposed light to the pupil in order from the object side, as shown in the four examples of FIGS. 1, 5, 9, and 13. It is composed of a pentaprism 2 and an eyepiece 3 which are optical members for converting an inverted image formed by the illustrated objective lens into an erect image.

  The eyepiece 3 includes, in order from the object side, a first lens group G1 including one lens L1 having a negative refractive power, and one lens L2 having a positive refractive power that performs diopter correction by moving in the optical axis direction. And a third lens group G3 having a positive refractive power.

  As a result, the principal point of the eyepiece 3 can be brought closer to the focusing screen, and the focal length of the eyepiece 3 can be shortened even when a sufficient distance from the focusing screen to the eyepiece 3 is ensured. A finder optical system having a high magnification can be obtained.

  The third lens group G3 includes a lens L3 having a positive refractive power in order from the object side, and a negative meniscus lens L4 having a concave surface directed toward the pupil side.

  By making the positive lens L3 of the third lens group G3 close to the second lens group G2, it becomes possible to suppress the refractive power of the second lens group G2, which is a movable group, and the second lens group G2 has a manufacturing error or the like. It is possible to prevent the diopter from changing drastically even if a slight lens position shift occurs. It is also possible to suppress the amount of aberration that occurs in the second lens group G2.

  Furthermore, the finder optical system of the present invention satisfies the conditional expressions (1) and (2).

  Conditional expression (1) represents the ratio of the focal length f1 of the first lens group G1 to the focal length f of the entire finder optical system. When the conditional expression (1) is satisfied, an appropriate refractive power can be obtained for diverging the light beam from the exit surface of the pentaprism 2.

  When the lower limit of conditional expression (1) is exceeded, the refractive power of the first lens group G1 becomes weak, and the light beam height at the exit surface of the pentaprism 2 needs to be increased. This leads to an increase in the size of the pentaprism 2, and it is difficult to sufficiently guide the luminous flux around the viewfinder to the eyepiece 3 and the eyepoint EP if it is attempted to be small. When the upper limit of conditional expression (1) is exceeded, the refractive power of the first lens group G1 becomes strong, and the height of the light beam emitted to the second lens group G2 increases, leading to an increase in the size of the eyepiece lens 3. In addition, it becomes difficult to correct astigmatism and coma.

  Conditional expression (2) represents the ratio of the focal length f3n of the negative meniscus lens L4 in the third lens group G3 to the focal length f of the entire finder optical system. By satisfying conditional expression (2), it is possible to ensure a sufficiently long eye relief while maintaining a high observation magnification.

  When the lower limit of conditional expression (2) is exceeded, the refractive power of the negative meniscus lens L4 in the third lens group G3 becomes weak, the main point position of the eyepiece 3 moves away from the focusing screen, and high observation magnification is maintained. It becomes difficult. If the upper limit of conditional expression (2) is exceeded, the refractive power of the negative meniscus lens L4 in the third lens group G3 will become strong, and it will be difficult to ensure a sufficiently long eye relief.

  Furthermore, in the finder optical system of the present invention, it is desirable that the object side surface of the positive lens L3 of the third lens group G3 be an aspherical surface and satisfy the conditional expression (3).

  Conditional expression (3) represents the aspherical shape of the positive lens L3 in the third lens group G3. By satisfying conditional expression (3), an increase in the diameter of the eyepiece lens can be suppressed.

  If the lower limit of conditional expression (3) is exceeded, the positive refractive power at the peripheral part of the aspheric surface becomes weak, and the height of the light beam at the peripheral part of the finder in the second lens group G2 becomes high. An attempt to secure a sufficient amount of light in the peripheral area will inevitably lead to an increase in the diameter of the pentaprism and eyepiece lens, and if it is attempted to be small, the light flux in the periphery of the viewfinder will be sufficiently guided to the eyepiece 3 and the eye EP It becomes difficult. If the upper limit of conditional expression (3) is exceeded, the positive refractive power at the peripheral part of the aspheric surface becomes strong, and the thickness deviation at the lens center and the peripheral part becomes large, so that the processing accuracy of the lens becomes poor.

  Furthermore, in the finder optical system of the present invention, it is desirable that the object side surface of the negative meniscus lens L4 of the third lens group G3 be an aspherical surface and satisfy the conditional expression (4).

  Conditional expression (4) represents the aspherical shape of the negative meniscus lens L4 in the third lens group G3. By satisfying conditional expression (4), it is possible to suppress the enlargement of the pentaprism 2.

  If the lower limit of conditional expression (4) is exceeded, the negative refractive power at the periphery of the aspheric surface becomes strong, the amount of change in the surface shape at the periphery of the lens increases, and the processing accuracy of the lens deteriorates. If the upper limit of conditional expression (4) is exceeded, the negative refractive power at the periphery of the aspheric surface becomes weak. With the increase in the size of the imaging device, it is necessary to increase the refractive power of the first lens group G1 in order to cope with a camera in which the peripheral light beam position at the focusing screen position tends to be high. Since the height of the light beam emitted to the lens group G2 is increased, the size of the eyepiece lens 3 is increased.

  Furthermore, in the finder optical system of the present invention, it is desirable that each lens constituting the eyepiece 3 is formed of a plastic material. Thereby, an inexpensive finder optical system can be provided. Further, molding and aspherical processing of each lens constituting the eyepiece lens are facilitated.

  Next, a lens configuration of an example according to the finder optical system of the present invention will be described. In the following description, the lens configuration is described in order from the object side to the pupil side.

  FIG. 1 is a lens configuration diagram of a finder optical system according to Example 1 of the present invention.

  The first lens group G1 of the eyepiece 3 includes a biconcave negative lens L1.

  The second lens group G2 of the eyepiece 3 is composed of a biconvex positive lens L2. The second lens group G2 moves along the optical axis during diopter correction.

  The third lens group G3 of the eyepiece 3 includes a meniscus positive lens L3 having a convex surface facing the object side, and a meniscus negative lens L4 having a concave surface facing the pupil side.

  In addition to the object side surface of the positive lens L3 and the object side surface of the negative lens L4 of the third lens group G3, the object side surface of the negative lens L1 of the first lens group G1 and the second lens group G2 The object side surface of the positive lens L2 has an aspherical shape.

  FIG. 5 is a lens configuration diagram of a finder optical system according to Example 2 of the present invention.

  The first lens group G1 of the eyepiece 3 includes a biconcave negative lens L1.

  The second lens group G2 of the eyepiece 3 is composed of a biconvex positive lens L2. The second lens group G2 moves along the optical axis during diopter correction.

  The third lens group G3 of the eyepiece 3 includes a meniscus positive lens L3 having a convex surface facing the object side, and a meniscus negative lens L4 having a concave surface facing the pupil side.

  In addition to the object side surface of the positive lens L3 and the object side surface of the negative lens L4 of the third lens group G3, the object side surface of the negative lens L1 of the first lens group G1 and the second lens group G2 The object side surface of the positive lens L2 has an aspherical shape.

  FIG. 9 is a lens configuration diagram of a finder optical system according to Example 3 of the present invention.

  The first lens group G1 of the eyepiece 3 includes a negative meniscus lens L1 having a convex surface facing the pupil side.

  The second lens group G2 of the eyepiece 3 is composed of a biconvex positive lens L2. The second lens group G2 moves along the optical axis during diopter correction.

  The third lens group G3 of the eyepiece 3 includes a biconvex positive lens L3 and a meniscus negative lens L4 with a concave surface facing the pupil.

In addition to the object side surface of the positive lens L3 and the object side surface of the negative lens L4 of the third lens group G3, the pupil side surface of the negative lens L1 of the first lens group G1 and the second lens group G2 The object side surface of the positive lens L2 has an aspherical shape.
ing.

  FIG. 13 is a lens configuration diagram of a finder optical system according to Example 4 of the present invention.

  The first lens group G1 of the eyepiece 3 includes a biconcave negative lens L1.

  The second lens group G2 of the eyepiece 3 is composed of a biconvex positive lens L2. The second lens group G2 moves along the optical axis during diopter correction.

  The third lens group G3 of the eyepiece 3 includes a meniscus positive lens L3 having a convex surface facing the object side, and a meniscus negative lens L4 having a concave surface facing the pupil side.

  Further, in addition to the object side surface of the positive lens L3 and the object side surface of the negative lens L4 of the third lens group G3, the object side surface and the pupil side surface of the negative lens L1 of the first lens group G1 The object side surface of the positive lens L2 of the second lens group G2 has an aspherical shape.

  Hereinafter, numerical examples corresponding to the above-described embodiments will be described.

  F in [focal length] represents a focal length.

  In the [lens specifications], the number of the first column is the lens surface number from the object side, the second column r is the radius of curvature of the lens surface, the third column d is the lens surface interval, and the fourth column nd is the d line ( The refractive index with respect to the wavelength λ = 587.56 mm), the fifth column νd indicates the Abbe number with respect to the d-line, and the sixth column indicates the ray height. * On the right side of the surface number indicates that the surface is aspherical.

  [Variable interval] indicates the value of each variable interval in diopter correction.

  [Conditional Expression Corresponding Value] indicates the value of each conditional expression in the embodiment.

  [Aspherical coefficient] indicates an aspherical coefficient that gives the aspherical shape of the lens surface marked with * in [Lens Specifications]. The shape of the aspheric surface is y for the displacement from the optical axis in the direction perpendicular to the optical axis, z for the displacement (sag amount) from the intersection of the aspheric surface and the optical axis in the optical axis direction, and r for the radius of curvature of the reference spherical surface. When the conic coefficient is K, 4, 6, 8, and the 10th-order aspheric coefficient is A4, A6, A8, and A10, the coordinates of the aspheric surface are expressed by the following equations.

  In addition, in the values of all the following specifications, the focal length f, the radius of curvature r, the lens surface interval d, and other length units described are in millimeters (mm) unless otherwise specified. In the system, the same optical performance can be obtained even in proportional expansion and proportional reduction, and the present invention is not limited to this. These reference numerals are the same in the other embodiments described below, and the description thereof is omitted.

  The aberration diagrams of the respective examples show aberrations when an ideal lens having a focal length of 15 mm is placed at the eye point position. ΔM and ΔS are a sagittal image surface and a meridional image surface, respectively, H is a pupil diameter, and Y is an image height.

Example 1
[Focal length]
f 54.49 58.64 50.43

[Lens specifications]
rd nd νd Effective ray height
[1] ∞ 0.3400
[2] ∞ 1.0000 1.49175 57.45
[3] ∞ 1.4600
[4] ∞ 67.3000 1.51680 64.20
[5] ∞ 3.3800
[6] * -41.9000 3.0000 1.63550 29.86 11.00
[7] 99.7500 d7 11.00
[8] * 17.9300 5.0000 1.49175 57.45 12.40
[9] -600.0000 d9 12.40
[10] * 35.5400 5.0000 1.49175 57.45 12.40
[11] 100.0000 2.1660 12.00
[12] * 22.2300 6.0000 1.63550 29.86 10.30
[13] 17.0000 18.0000 7.90
[14] ∞ 3.50

[Variable interval]
Diopter -1.02 -3.14 1.53
d7 2.5892 1.0000 4.3726
d9 2.7833 4.3726 1.0000

[Values for conditional expressions]
Conditional expression (1) Conditional expression (2) Conditional expression (3) Conditional expression (4)
f1 / f f3n / f Hmax / sag3p Hmax / sag3n
Condition range -1.1 <x <-0.6 -4.0 <x <-1.2 -9 <x <-5 8.5 <x <12.0
Example 1 -0.92 -3.93 -5.32 9.39

[Aspheric coefficient]
6th surface 8th surface 10th surface 12th surface
K 10.9882 -1.0409 2.8137 -6.8031
A4 2.4639E-05 -2.3143E-05 4.7643E-05 1.2737E-05
A6 -3.3857E-08 5.7454E-08 1.7524E-07 -6.4461E-07
A8 1.0557E-09 -8.4998E-10 -8.4098E-10 1.7695E-10
A10 5.2375E-12 9.0114E-12 9.4407E-12

Example 2
[Focal length]
f 54.50 58.48 50.55

[Lens specifications]
rd nd νd Effective ray height
[1] ∞ 0.3400
[2] ∞ 1.0000 1.49175 57.45
[3] ∞ 1.4600
[4] ∞ 67.3000 1.51680 64.20
[5] ∞ 3.1500
[6] * -48.9900 4.1900 1.58250 29.86 11.00
[7] 80.9600 d7 11.00
[8] * 17.7300 5.0000 1.49175 57.45 12.20
[9] -600.0000 d9 12.20
[10] * 33.3700 5.0000 1.49175 57.45 12.00
[11] 100.0000 1.1800 11.50
[12] * 24.1900 6.0000 1.58250 29.86 10.40
[13] 17.0000 18.0000 7.90
[14] ∞ 3.50

[Variable interval]
Diopter -1.02 -3.15 1.53
d7 2.6061 1.0000 4.4253
d9 2.8192 4.4253 1.0000

[Values for conditional expressions]
Conditional expression (1) Conditional expression (2) Conditional expression (3) Conditional expression (4)
f1 / f f3n / f Hmax / sag3p Hmax / sag3n
Condition range -1.1 <x <-0.6 -4.0 <x <-1.2 -9 <x <-5 8.5 <x <12.0
Example 2 -0.95 -2.59 -5.26 8.91

[Aspheric coefficient]
6th surface 8th surface 10th surface 12th surface
K 14.933 -1.0768 4.8955 -6.654
A4 2.3851E-05 -2.1722E-05 4.2557E-05 -7.5204E-06
A6 -1.2892E-07 1.0536E-07 2.0221E-07 -6.5347E-07
A8 2.0953E-09 -1.1745E-09 -1.5784E-09 1.3063E-09
A10 2.5906E-13 1.3025E-11 5.2747E-12

Example 3
[Focal length]
f 54.32 57.90 50.71

[Lens origin]
rd nd νd Effective ray height
[1] ∞ 0.3400
[2] ∞ 1.0000 1.49175 57.45
[3] ∞ 1.4600
[4] ∞ 67.3000 1.51680 64.20
[5] ∞ 4.1700
[6] -32.0900 2.0000 1.58250 29.86 11.70
[7] * -47551.8100 d7 12.20
[8] * 20.6400 7.0500 1.49175 57.45 13.40
[9] -74.6200 d9 13.40
[10] * 45.6100 4.8900 1.49175 57.45 12.00
[11] -184.0000 0.3000 12.00
[12] * 28.0000 6.0000 1.58250 29.86 11.00
[13] 15.2900 19.5000 8.20
[14] ∞ 3.50

[Variable interval]
Diopter -1.02 -3.15 1.53
d7 2.6710 1.0000 4.5853
d9 2.9143 4.5853 1.0000

[Values for conditional expressions]
Conditional expression (1) Conditional expression (2) Conditional expression (3) Conditional expression (4)
f1 / f f3n / f Hmax / sag3p Hmax / sag3n
Condition range -1.1 <x <-0.6 -4.0 <x <-1.2 -9 <x <-5 8.5 <x <12.0
Example 3 -1.01 -1.29 -8.86 11.73

[Aspheric coefficient]
7th surface 8th surface 10th surface 12th surface
K 0.0000 -1.0197 1.2095 -3.0019
A4 -1.0116E-05 -4.8217E-05 7.8524E-05 -3.9593E-05
A6 1.3611E-07 2.6034E-07 -1.7753E-07 -2.4481E-07
A8 -4.9825E-10 -6.9572E-10 5.0938E-10 1.4535E-09

Example 4
[Focal length]
f 54.47 59.19 49.99

[Lens specifications]
rd nd νd Effective ray height
[1] ∞ 0.3400
[2] ∞ 1.0000 1.49175 57.45
[3] ∞ 1.4600
[4] ∞ 67.3000 1.51680 64.20
[5] ∞ 3.7600
[6] * -28.5100 2.3500 1.58250 29.86 11.20
[7] * 99.1900 d7 11.20
[8] * 18.8800 6.0000 1.49175 57.45 12.70
[9] -84.9500 d9 12.70
[10] * 23.5900 6.6000 1.49175 57.45 12.00
[11] 183.1600 1.0000 11.50
[12] * 32.5700 5.8000 1.58250 29.86 10.50
[13] 17.0000 18.0000 7.90
[14] ∞ 3.50

[Variable interval]
Diopter -1.01 -3.14 1.54
d7 2.1764 1.0000 3.4809
d9 2.3045 3.4809 1.0000

[Values for conditional expressions]
Conditional expression (1) Conditional expression (2) Conditional expression (3) Conditional expression (4)
f1 / f f3n / f Hmax / sag3p Hmax / sag3n
Condition range -1.1 <x <-0.6 -4.0 <x <-1.2 -9 <x <-5 8.5 <x <12.0
Example 4 -0.69 -1.29 -7.62 9.88

[Aspheric coefficient]
6th surface 7th surface 8th surface 10th surface 12th surface
K 3.8769 24.8459 -1.0421 -0.2133 -28.6734
A4 2.7604E-05 1.7089E-06 -2.3980E-05 4.9740E-05 2.9958E-05
A6 4.9126E-08 6.3108E-08 2.0602E-07 -1.0438E-07 -1.0362E-06
A8 2.6138E-09 7.8731E-10 -1.6951E-09 2.0645E-09 3.0890E-09
A10 3.3611E-12 3.4128E-13 6.3937E-12

DESCRIPTION OF SYMBOLS 1 SI board 2 Penta prism 3 Eyepiece L1 1st lens L2 2nd lens L3 3rd lens L4 4th lens G1 1st lens group G2 2nd lens group G3 3rd lens group EP Eye point

Claims (4)

In the finder optical system for observing the image formed by the objective lens with the eyepiece through the optical system for erect image formation,
The eyepiece is a first lens group composed of a single lens having a negative refractive power in order from the object side, and a first lens composed of a single lens having a positive refractive power that performs diopter correction by moving in the optical axis direction. It consists of two lens groups and a third lens group with positive refractive power,
The third lens group includes a lens having a positive refractive power in order from the object side and a negative meniscus lens having a concave surface facing the pupil side, and satisfies the following conditional expression.
(1) -1.1 <f1 / f <-0.6
(2) -4.0 <f3n / f <-1.2
However, the focal length of the first lens group is f1, the focal length of the negative meniscus lens of the third group is f3n, and the focal length of the entire finder optical system when the diopter is -1 pt is f. 2. The finder optical system according to claim 1, wherein the object side surface of the positive lens of the third lens group is an aspherical surface, and satisfies the following conditional expression.
(3) -9.0 <Hmax / sag3p <-5.0
However, the maximum ray height on the aspheric surface side of the positive lens of the third lens group is Hmax, and the deviation in the optical axis direction between the paraxial spherical surface and the aspheric surface at the maximum ray height Hmax on the aspheric surface side of the positive lens is sag3p. . 3. The finder optical system according to claim 1, wherein an object-side surface of the negative meniscus lens of the third lens group is an aspheric surface, and satisfies the following conditional expression.
(4) 8.5 <Hmax / sag3n <12.0
However, the maximum ray height on the aspherical side of the negative meniscus lens of the third lens group is Hmax, and the deviation in the optical axis direction between the paraxial spherical surface and the aspherical surface at the maximum ray height Hmax on the aspherical side of the negative meniscus lens is sag3n. And   The finder optical system according to claim 1, 2 or 3, wherein each lens constituting the eyepiece is made of a plastic material.

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