JPH0830786B2 - Albada type reverse Galileo variable magnification - Google Patents

Description

The present invention has, in order from the subject side, an objective lens group having a negative refractive power and an eyepiece lens group having a positive refractive power, and further an albada-type infinder such as a visual field frame. The present invention relates to an albada-type reverse Galileo finder capable of displaying.

Conventionally, various types of finder optical systems for lens shutter cameras have been proposed and put to practical use. Here, the lens shutter camera is a great selling point because of its compactness, and therefore the finder optical system is also required to be compact. Therefore, the above-mentioned albada-type reverse Galileo finder optical system does not require a space different from that of the finder optical system to display the field-of-view frame and the distance-measuring frame in the viewfinder, so that a compact finder optical system is configured. Often used because it has the advantage of being able to

Further, in recent years, various cameras have been proposed in which the focal length of the photographing lens can be switched, and for this purpose, it is necessary to switch the range of the subject displayed by the visual field frame indicating the shooting range. Therefore, various zooming methods have been proposed in which the finder magnification is switched by using the above-mentioned Albada type Galileo finder so as to correspond to the switching of the focal length in the photographing lens of the camera.

For example, at least a part of the objective lens group is configured to be movable in the optical axis direction, a method of continuously changing the finder magnification by this movement, and a part of the objective lens group can be moved in the optical axis direction. In addition, a method is known in which the remainder is moved in and out of the optical path of the finder, and the finder magnification is switched by moving and moving the objective lens group in the optical axis direction. However, according to the former method, the lens elements that make up the finder optical system do not change before and after zooming, so the outer diameter of the lens on the most subject side becomes extremely larger than the outer diameter of the following lenses, It has a drawback that the viewfinder optical system is prevented from being made compact.

Therefore, an example of adopting the latter method will be shown. 11 (A) and (B) are longitudinal sectional views of the albada-type reverse Galileo finder optical system proposed by the applicant of the present invention in Japanese Patent Application No. 60-110973 (filing date: May 23, 1985). And then
FIG. 11 (A) shows the high magnification state, and FIG. 11 (B) shows the low magnification state. As shown in FIGS. 11A and 11B, this finder optical system includes, in order from the subject side, a first lens (G ₁ ) having a negative refractive power and a second lens (G ₁ ) having a negative refractive power. ₂ ), a third lens (G ₃ ) having a positive refractive power and a pupil side surface being a reflection surface for Albada, and a fourth lens (G ₄ ) having a positive refractive power. (G ₅ ) is a field frame plate having a field frame (Fr) vapor-deposited on its subject side surface. Then, from the low magnification state shown in FIG. 11 (B), the second lens (G ₂ ) is retracted out of the finder optical path, and the first lens (G ₁ ) is moved rearward in the optical axis direction. The high magnification state shown in FIG.

According to this method, the outer diameter of each lens element can be made approximately the same as in the case of an inverse Galileo finder that does not perform magnification, but the total length of the finder is limited by the thickness of the camera. In order to ensure the above, the effective diameter of the viewfinder window of the camera provided on the object side of the objective lens becomes large to some extent.

Here, this discussion is about the case where the size of the view frame displayed in the viewfinder by the Albada finder optical system does not change before and after the magnification change. Therefore, instead of changing the finder magnification, there is also proposed a method in which the size of the field frame or the refractive power of the objective lens group and the method of changing the size of the field of view are combined. Since the luminous flux that is formed changes the viewing angle that is applied to the eye,
It has a drawback that the finder image becomes difficult to see.

Therefore, an object of the present invention is to reduce the finder magnification in an albada-type reverse Galilean finder that switches the finder magnification by switching the objective lens group while keeping the image of the field of view frame by the Albada finder optical system constant before and after zooming. It is an object of the present invention to provide a viewfinder optical system for a camera that can obtain a zoom ratio of 1.4 or more without reducing the effective diameter of the viewfinder optical system.

In order to achieve the above object, the present invention is shown in FIG.
As schematically shown in (B), from the subject side,
The first objective lens (L ₁ ) having negative refractive power, the second objective lens (L ₂ ) having negative refractive power, and the eyepiece lens (L ₃ ) having positive refractive power, An albada-type reverse Galilean variable magnification finder in which the finder magnification can be switched by moving _{one of} the second objective lenses (L ₁ ) and (L ₂ ) into and out of the finder optical path and moving the other in the optical axis direction, Eyepiece (L ₃ )
The subject side surface (r ₅ ) of the object is a reflecting surface for Albada, and the following conditions are satisfied.

(1) 0.3 <d / L _W <0.6 where d is the core thickness of the eyepiece lens (L ₃ ), and L _W is the total length of the finder in the low magnification state. Incidentally, FIG. 1 (A)
In (B), (L ₄ ) is a visual field frame plate in which a visual field frame (Fr) is vapor-deposited on the surface (r ₇ ) on the subject side.

Hereinafter, the present invention will be described in detail. First, when designing a finder optical system for a lens shutter camera in which the focal length of the taking lens can be switched, it is sufficient that the zoom ratio of the finder magnification substantially matches the rate of change of the focal length of the taking lens. That is, it suffices if the viewfinder image expresses a state that is almost the same as that caused by a change in the photographing magnification on the film surface. Therefore, there is a certain degree of freedom in the absolute value of the finder magnification itself. On the other hand, in a lens shutter camera with a switchable focal length, the rate of change of the focal length of the taking lens required by the user is considered to be 1.4 times or more. For example, the change rate is 1.43 when the focal length of the taking lens can be switched between 35 mm and 50 mm, and 1.4 when it can be switched between 50 mm and 70 mm. Therefore, in the present invention,
Consider a case where the magnification ratio m of the finder magnification is m ≧ 1.4 (A). However, when the finder magnification in the high magnification state is Γt and the finder magnification in the low magnification state is Γs, m = Γt / Γs (B).

As apparent from a comparison between FIGS. 1 (A) and (B) and FIGS. 11 (A) and (B), the present invention is based on the conventional albada-type reverse Galileo finder shown in FIGS. The air gap between the third lens (G ₃ ) and the fourth lens (G ₄ ) of the optical system is eliminated, and the gap is filled with an optical medium. In the present invention, by making the effective optical path length of the finder optical system shorter than that of the conventional example, the effective diameter of the finder optical system can be made smaller while maintaining the finder magnification and the total length of the finder that are substantially equal to those of the conventional example. Can be done. Here, in the configuration of the conventional example shown in FIGS. 11A and 11B, even if a parallel plate optical member is arranged between the third lens (G ₃ ) and the fourth lens (G ₄ ). Although similar effects can be obtained, in the present invention, a lens having a reflecting surface for Arbata (FIGS. 11A and 11B is a third lens (G ₃ ) in the conventional example) and an eyepiece lens (the fourth lens). (G ₄ )) is integrated, so the number of lenses can be reduced.

The condition (1) defines the core thickness of the eyepiece lens (L ₃ ) in relation to the total length of the viewfinder. Condition (1)
If the core thickness of the eyepiece lens (L ₃ ) becomes smaller than the lower limit of, the reflecting surface for albada formed on the surface (r ₅ ) on the subject side will be too close to the observer's eye, so in the optical system for albada If the diopter of the viewfinder display image is kept at an appropriate value (for example, -1 diopter), the astigmatic difference in the optical system that forms the viewfinder image increases significantly and it becomes difficult to correct it. It is also difficult to reduce the diameter. On the other hand, when the core thickness of the eyepiece lens exceeds the upper limit of the condition (1), the objective lens that can be moved in the optical axis direction for zooming (the first objective lens (see FIGS. 1A and 1B) The moving distance of L ₁ )) is insufficient, and it becomes difficult to obtain the variable power ratio of m ≧ 1.4 as described above. Even when the total length of the finder is shortened beyond the upper limit of the condition (1), the moving distance of the objective lens for zooming becomes short and it becomes difficult to obtain a desired zoom ratio. .

As described above, in order to further increase the viewfinder magnification while maintaining compactness by the condition (1),
It is desirable to satisfy the following conditions.

(2) 0.35 <| Lt / rb | <0.9 where Lt is the total length of the finder in the high magnification state, and rb is the radius of curvature of the pupil side surface of the eyepiece lens (L ₃ ).

To simplify the explanation, considering the finder total length Lt in the high magnification state as fixed, exceeding the upper limit of the condition (2) means that the radius of curvature of the pupil-side surface (r ₆ ) of the eyepiece (L ₃ ) is exceeded. It means that the absolute value of | rb | becomes smaller than a predetermined value. In this way, when the curvature of the pupil-side surface (r ₆ ) of the eyepiece lens (L ₃ ) becomes strong, it is attempted to keep the diopter of the finder display image in the albada optical system at an appropriate value (eg 1-diopter). Then, the astigmatic difference in the Albada type optical system remarkably increases and it becomes difficult to correct it. Further, in order to make the diopter of the viewfinder image substantially match the diopter of the image displayed in the viewfinder, the refractive powers of the first objective lens (L ₁ ) and the second objective lens (L ₂ ) are both strong, and the high magnification is high. In this state, the finder magnification Γt decreases, and it becomes difficult to obtain the desired zoom ratio.

On the contrary, if the total finder length Lt in the high magnification state exceeds the upper limit of the condition (2), the total finder length L _W in the low magnification state also increases in order to obtain the desired zoom ratio. Becomes large and the compactness is impaired. More specifically, if the core thickness of the eyepiece lens (L ₃ ) is fixed and the core thickness of the first objective lens (L ₁ ) is increased to increase the finder total length Lt in the high magnification state, the finder magnification in the high magnification state is increased. Since Γt decreases, the position of the first objective lens (L ₁ ) in the low magnification state should be set to the eyepiece lens (L ₃ ) in order to obtain the desired zoom ratio.
Need to keep away from. This means that the finder total length L _W in the low magnification state is increased, and the compactness is impaired. The same applies when the core thickness of the eyepiece lens (L ₃ ) is increased to increase the total length of the finder in the high magnification state.

Further, if | rb | becomes larger and exceeds the lower limit of the condition (2), the refracting power of the pupil side surface (r ₆ ) of the eyepiece lens (L ₃ ) becomes small. Surface (r ₅ ) becomes too strong, the curvature of field in the albada optical system becomes large, and the astigmatic difference also becomes significantly large, making it difficult to correct it and making the viewfinder display image very difficult to see. . Further, when the total finder length Lt in the high magnification state becomes smaller and exceeds the lower limit of the condition (2), the effect of reducing the effective diameter of the finder optical system, which is the object of the present invention, becomes insufficient.

Furthermore, in the present invention, by introducing an aspherical surface into at least one of the first and second objective lenses, it is possible to reduce the field curvature and astigmatic difference of the finder optical system.

Further, in the present invention, at least one of the eyepieces
By introducing an aspherical surface, it is possible to further reduce the field curvature and distortion in the Albada optical system.

Hereinafter, examples of the present invention will be shown in a table together with comparative examples. Here, the comparative example is Example 1 described in Japanese Patent Application No. 60-110973. In the table, the refractive index (Nd) and the Abbe number (νd) are values for the d-line. The surface marked with (*) indicates an aspherical surface, and its shape is expressed as follows.

Here, X is the distance in the optical axis direction from the aspherical vertex,
Y is the distance in the direction perpendicular to the optical axis from the optical axis, C ₀ is the reference curvature, ε is the quadric surface parameter, and Ai is the i-th order aspherical curvature.

Incidentally, in Example 1 described later, the eyepiece lens (L ₃ )
The eye-side surface (r ₆ ) of the frame has marks to be displayed in the viewfinder such as a field-of-view frame and distance-measuring frame, and the field-of-view frame plate (L ₄ ) shown in FIGS. 1A and 1B is omitted. Has been done. According to this structure, the field frame plate (L ₄ ) can be omitted to reduce the number of parts constituting the finder optical system. In Examples 2 to 4,
Similar to FIGS. 1 (A) and 1 (B), a field frame plate (L ₄ ) made of parallel flat plate glass is arranged on the pupil side of the eyepiece lens (L ₃ ) and its object side surface (r ₇ ) Is provided with the above mark. By arranging the visual field frame plate (L ₄ ) on the pupil side of the eyepiece lens (L ₃ ) in this way, it is possible to protect the viewfinder optical system from the pupil side and obtain a dustproof effect. Furthermore, in all the examples, the object-side surface (r ₅ ) of the eyepiece lens (L ₃ ) is used as a reflective surface for albada, and this reflective surface partially or semi-transparents the lens surface. It may be a surface or a part of it may be a total reflection surface. Next, the effect of the present invention will be described. An object of the present invention is to reduce the effective diameter of the finder optical system, and the term "effective diameter" here means the maximum effective diameter of the entire finder optical system. However, in the case of the reverse Galileo finder optical system, its effective diameter changes according to the total finder length, the finder magnification, the angle of view of the photographing lens, and the set pupil diameter. Generally, as the finder magnification increases, the effective diameter inevitably increases, and the effective diameter also increases even if the total finder length and the angle of view of the photographing lens increase.

Therefore, when the finder magnification is Γ and the finder total length is L, a ray reaching the center of the pupil (so-called chief ray)
Let h be the maximum value of the optical path height that intersects the lens element of the finder optical system, and let D = h / Γ · L (D) be an index representing the maximum optical path height of the finder optical system. Here, this maximum optical path height D is the maximum value h of the optical path height of the chief ray standardized by them so that the effective diameters of the finder optical systems having different finder magnifications Γ and different finder overall lengths L can be compared. . 2 (A) and 2 (B) show the maximum value ht of the optical path height in the high magnification state and the finder total length Lt, the maximum value hw of the optical path height in the low magnification state and the finder total length Lw in the present invention, respectively. . FIG. 2 (A) shows a high magnification state, and FIG. 2 (B) shows a low magnification state.

Next, the maximum optical path height Dt in the high magnification state and the maximum optical path height Dw in the low magnification state in Examples 1 to 4 of the present invention and Comparative Example
It is shown in Table 6 in relation to the angle of view (tan θ). Here, the angle of view (tan θ) indicates a half angle of view for a diagonal length of 100% on the film surface in the lens shutter camera, and in the high magnification state, the focal length f of the photographing lens is f = 50 mm, 60 mm, 70 mm, 8
It corresponds to 0 mm respectively, and in the low magnification state it corresponds to the focal length f of the taking lens f = 35 mm, 40 mm and 50 mm, respectively. Further, in Table 6, the maximum optical path height Dt of the comparative example or
Maximum optical path height Dt of Examples 1 to 4 of the present invention with respect to Dw or
The reduction rate of Dw is also shown.

As is clear from Table 6, in the examples of the present invention, the optical path height can be reduced by up to about 30% of the comparative examples, that is, the effective diameter of the finder optical system can be reduced. Further, in the present invention, the number of parts constituting the finder optical system can be reduced as compared with the comparative example, and simplification of assembly / adjustment can be realized and cost reduction can be achieved.

[Brief description of drawings]

1 (A) and 1 (B) are cross-sectional views schematically showing lens arrangements of an albada-type inverse Galileo variable power finder according to the present invention in a high magnification state and a low magnification state, respectively, and FIG. 2 (A).
FIG. 3B is a sectional view showing the maximum value of the optical path height and the total length of the finder, and FIGS. 3A and 3B are the first embodiment of the present invention.
4A and 4B are sectional views showing the lens arrangements in the high magnification state and the low magnification state, respectively, FIGS. 4A and 4B are aberration diagrams showing various aberrations in the high magnification state and the low magnification state, respectively, and FIG. 5A.
FIG. 6B is a sectional view showing the lens arrangement of Embodiment 2 of the present invention in a high magnification state and a low magnification state, and FIGS. 6A and 6B.
FIG. 7A is an aberration diagram showing various aberrations in the high magnification state and the low magnification state, and FIGS. 7A and 7B are sectional views showing lens arrangements in the high magnification state and the low magnification state of Example 3 of the present invention, respectively. 8 (A) and 8 (B) are aberration diagrams showing various aberrations in the high magnification state and the low magnification state, respectively, and FIGS. 9 (A) and 9 (B).
Is a cross-sectional view showing the lens arrangement in a high magnification state and a low magnification state of Example 4 of the present invention, and FIGS. 10A and 10B are aberration diagrams showing various aberrations in the high magnification state and the low magnification state, respectively. 11 (A) and 11 (B) are cross-sectional views showing lens arrangements in a high-magnification state and a low-magnification state, respectively, in a conventional Albada type reverse Galileo variable magnification finder. (L ₁ ): first objective lens, (L ₂ ): second objective lens, (L ₃ ): eyepiece lens.

 ─────────────────────────────────────────────────── ─── Continuation of the front page Examiner Katsuhisa Segawa (56) References JP-A-60-166934 (JP, A) JP-A-61-267727 (JP, A) JP-A-60-57329 (JP, A)

Claims (4)

[Claims] 1. A first objective lens having a negative refracting power, a second objective lens having a negative refracting power, and an eyepiece lens having a positive refracting power in order from the subject side. An albada-type reverse Galilean variable magnification finder in which one of the objective lenses is moved in and out of the finder optical path and the other is moved in the optical axis direction, whereby the viewfinder magnification is switched. Albada-type inverse Galileo variable magnification finder characterized by satisfying the following conditions as well as a reflecting surface for use: 0.3 <d / L _W <0.6 where d: core thickness of eyepiece lens, L _W : low magnification Finder total length in the state. 2. An albada-type inverse Galileo variable magnification finder according to claim 1, further satisfying the following condition: 0.35 <| Lt / rb | <0.9 where Lt: high magnification Finder total length in the state, rb: radius of curvature of the pupil-side surface of the eyepiece. 3. The albada-type inverse Galileo variable magnification finder according to claim 1, wherein at least one surface of either of the first and second objective lenses is an aspherical surface. 4. The alibada-type inverse Galileo variable power finder according to claim 1, wherein at least one surface of the eyepiece lens is an aspherical surface.