JPH0425688Y2 - - Google Patents

Description

[Detailed explanation of the idea]

(Industrial Application Field) The present invention relates to a variable magnification finder, and in particular to a variable magnification finder with a simple structure suitable for use as a finder in photographic cameras, video cameras, etc., which maintains good optical performance before and after changing the magnification of the finder. It is related to. (Prior art) Conventionally, in cameras where the photographing system and viewfinder system are configured separately, when the photographing system is a variable magnification system, the magnification of the viewfinder's field of view, that is, the display of the shooting range changes as the magnification changes. A viewfinder is preferable for photography. Conventionally, known means for switching the photographing range display include (a) a method of changing the size of the field of view frame, and (b) a method of changing the magnification of the viewfinder. As a method of changing the size of the visual field frame, for example, as in Japanese Patent Application Laid-Open No. 161931/1984, the apparent size of the visual field frame is changed by moving the visual field frame for forming the visual field frame image. There are known methods to do this. However, in this case, when the viewfinder has a high magnification, that is, when the field of view of the viewfinder becomes narrow, the apparent field of view of the field frame becomes small, resulting in a viewfinder that is extremely difficult for the observer to see. On the other hand, in the case of a method of converting the finder magnification itself, for example,
- As in Publication No. 168738, the method of converting the finder magnification by replacing the objective lens constituting the Albada type finder with an objective lens having a different refractive power keeps the apparent field of view of the shooting range constant. This allows the observer to perform viewfinder observation with a feeling closer to that of actual photography, and can be said to be superior to the method of changing the size of the field of view frame. In addition, a real image finder that observes a real finder image formed by an objective lens with positive refractive power, the so-called Kepler type finder, can have a field mask at the real image position, and the outline of the finder field of view is clear and the eye It has the advantage that the viewfinder field of view does not change even if the position of the finder changes, so it is often used as a variable magnification finder. However, since the Kepler type finder forms a real finder image, the finder system tends to be large in size, and the finder system tends to become complicated in order to maintain good optical performance. This tendency was particularly noticeable in the Kepler type finder which has a variable magnification function. (Problems to be solved by the invention) The present invention moves some of the lenses on the finder optical axis in a real image finder, and also attaches and detaches an auxiliary lens to a predetermined position on the finder optical axis. To provide a Keplerian type variable magnification finder having a simple structure, easily obtaining finder magnification and maintaining good optical performance before and after variable magnification. (Means for solving the problem) An objective lens made of a single lens having positive refractive power and an eyepiece lens having positive refractive power, and an image formed by the objective lens is observed with the eyepiece lens. In the finder, the objective lens is moved toward the image plane side, and an auxiliary lens consisting of a single lens having positive refractive power is attached on the optical axis on the object side of the objective lens to lower the finder magnification. A variable magnification finder characterized by variable magnification and satisfying the following conditions. 0≦f ₀ /R ₃ ≦2.0 where f ₀ is the focal length of the objective lens, and R ₃ is the radius of curvature of the objective lens on the object side. (Embodiment) FIGS. 1A and 1B are schematic diagrams of an optical system of a variable magnification viewfinder according to an embodiment of the present invention. Figure A shows the optical arrangement before the finder magnification is changed, ie, at high magnification, and Figure B shows the optical arrangement after the finder magnification is changed, ie, at low magnification. In the figure, 1 is a protective glass, and 2 is an objective lens, which in this embodiment is composed of one positive lens. 3
4 is a field lens, and 4 is a Porro prism that creates inverse images of the top, bottom, left and right finder images, shown as a glass block unfolded for simplicity. 5 is an eyepiece with positive refractive power, and 6 is a pupil position for observation.
M is a field frame indicating a shooting range, and is provided between the field lens 3 and the Porro prism 4. In this embodiment, the protective glass 1 is
A finder image is formed near the field frame M through the field lens 3. After this finder image is reversed vertically and horizontally using a Porro prism 4, it is observed through an eyepiece lens 5. And the viewfinder magnification is changed by the objective lens 2.
The magnification is changed to a low magnification by moving the lens toward the image plane side as shown by the arrow and attaching an auxiliary lens 7 with a positive refractive power on the front optical axis of the objective lens 2 as shown in FIG. 1B. There is. FIGS. 2A and 2B are schematic diagrams showing the paraxial refractive power arrangement of the objective lens 2, eyepiece lens 5, and auxiliary lens 7 at this time. Figure A is before the magnification change, and Figure B is after the magnification change. In this embodiment, the finder magnification γ is as shown in FIGS. 2A and 2B, when the finder system is a complete afocal real image finder and the refractive powers of the objective lens 2 and the eyepiece lens 5 are φ ₁ and φ ₂ , respectively. γ= It is expressed as φ ₂ /φ ₁ . In this embodiment, an auxiliary lens 7 with a positive refractive power φ _A is mounted on the front optical axis of the objective lens 2, and by increasing the refractive power φ ₁ of the objective lens 2, the viewfinder magnification is lowered. . At this time, if the objective lens is composed of a plurality of lenses, it is best to renormalize all the lenses or at least one positive lens A, and then attach an auxiliary lens to reduce aberration fluctuations before and after changing the finder magnification. This is preferable because it allows for In addition, in this example, in order to mainly correct off-axis aberrations well, the radius of curvature of the lens surface closest to the object side of the objective lens having positive refractive power is set to R3, and the focal length of the entire objective lens is set to f. When it is ₀ , it is desirable that 0≦f ₀ /R3≦2.0. If the upper limit of the above formula is exceeded, the image plane characteristics will be under-corrected, resulting in a large astigmatism difference, and if the lower limit is exceeded, the image surface characteristics will be over-corrected, resulting in a large astigmatism difference, which is good. This is undesirable because it becomes difficult to correct off-axis aberrations. In the embodiment shown in FIGS. 1A and 1B, the removable auxiliary lens is a single positive lens 1 whose both lens surfaces are convex.
Although this auxiliary lens is made up of a plurality of lenses, it may be made up of a plurality of lenses. Further, although the eyepiece lens is constructed by symmetrically arranging two lenses of the same shape to correct aberrations, it may be constructed of a plurality of lenses having completely different shapes. In this embodiment, since the position of the field frame M remains unchanged when the viewfinder is changed in magnification, only the size of the viewfinder field image changes and the size of the field frame does not change, so that the apparent field of view of the shooting range is kept constant. can do. Next, numerical examples of the present invention will be shown. In the numerical examples, Ri is the radius of curvature of the i-th lens surface from the object side, Di and di are the thickness and air spacing of the i-th lens surface from the object side, and Ni and νi are the curvature radius of the i-th lens surface from the object side, respectively. These are the refractive index and Atsube number of the lens glass. The aspherical shape has the X axis in the optical axis direction, the H axis in the direction perpendicular to the optical axis, the direction of light travel as positive, R as the paraxial radius of curvature, and A, B, C, D, and E as the aspherical coefficients. time It is expressed by the formula. Note that R1 and R2 are protective glasses, R5 and R6 are field lenses, R7 and R8 are Porro prisms, and R1
3 is the eyes.

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[Table] (Effects of the invention) As described above, according to the invention, at least one positive lens of the objective lens is retracted and an auxiliary lens with positive refractive power is mounted on the front optical axis of the objective lens. It is possible to achieve a variable magnification finder that is particularly suitable for a real-image type finder system and can easily obtain a predetermined variable magnification ratio while maintaining good imaging performance of finder images before and after magnification.

[Brief explanation of the drawing]

Figures 1A and B are schematic diagrams of the optical system before and after zooming of a variable magnification finder according to an embodiment of the present invention, and Figures 2A and B are paraxial refraction diagrams of the variable magnification finder of the present invention before and after zooming. FIGS. 3A and 3B, which are explanatory diagrams showing force distribution, are various aberration diagrams for an object at infinity before and after zooming in a numerical embodiment of the present invention. In the aberration diagram, ΔM is the meridional image plane and ΔS is the sagittal image plane. In the figure, 2 is an objective lens, 3 is a field lens,
4 is a developed view of the Porro prism, 5 is the eyepiece, 7
is an auxiliary lens.

Claims (1)

[Claims for Utility Model Registration] An objective lens consisting of a single lens having positive refractive power and an eyepiece lens having positive refractive power, and an image formed by the objective lens is observed with the eyepiece lens. In the finder, the objective lens is moved toward the image plane side, and an auxiliary lens consisting of a single lens having positive refractive power is attached on the optical axis on the object side of the objective lens to change the finder magnification to a low magnification. A variable magnification finder which is characterized by magnifying power and satisfying the following conditions. 0≦f ₀ /R ₃ ≦2.0 where f ₀ is the focal length of the objective lens, and R ₃ is the radius of curvature of the objective lens on the object side.