Eyepiece optical system for a single lens reflex camera

Photography – With viewfinder – Lens detail

Reexamination Certificate

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C359S645000

Reexamination Certificate

active

06253033

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an eyepiece optical system for a single lens reflex camera.
2. Description of the Related Art
In a single lens reflex camera, an erecting optical system using a hollow penta mirror instead of a penta prism is well known. Since a penta mirror has the reflection-optical-path in the air, the equivalent air thickness along the optical axis cannot be made shorter up to 1
times (n: the refractive index of a penta prism to be compared) of the equivalent air thickness of the penta prism. Conversely, since the reflection-optical-path is ‘n’ times as long as that of the penta prism having the same geometrical distance of the reflection-optical-path, the focal length of an eyepiece optical system is made longer, and a finder magnification is lowered. Accordingly, a special design is required for the eyepiece optical system. For example, Japanese Unexamined Patent Publication Nos. 9-146019, and U.S. Pat. No. 5,136,427 propose a two-lens-element eyepiece optical system constituted by a positive lens element and a negative lens element, in this order from the side of a penta mirror.
However, the eyepiece optical system in Japanese Unexamined Patent Publication No.9-146019 is for a single lens reflex camera which uses a film type smaller than a 135-type film, more concretely, for an APS (advanced photo system) camera. Therefore the eyepiece optical system cannot be converted to an eyepiece optical system for a single lens reflex camera in which the a 135-type film is used. Furthermore, the eyepiece optical systems in and U.S. Pat. No. 5,136,427 is not provided with an optical system having a diopter adjusting function. Of course it is theoretically possible to change the diopter by moving at least a portion of lens elements constituting an eyepiece optical system. However, since an optical system having a diopter adjusting function is not originally considered, if an attempt is made to satisfy a condition for maintaining a sufficient eye relief (the distance from the final surface, at the side of a photographer's eye, of the eyepiece optical system to the eyepoint) with respect to the light rays from the periphery of the field-of-view, a range of adjustment is smaller even if the lens diameter is increased, whereby an effect on a diopter adjustment is insufficient.
Furthermore, U.S. Pat. No. 5,313,327 proposes an eyepiece optical system for a penta mirror, the eyepiece optical system of which includes a positive single lens element. Also, this eyepiece optical system can theoretically change the diopter by moving the positive single lens element. However, since the amount of change in the diopter is small with respect to the amount of movement of the positive single lens element, a sufficient range of diopter adjustment cannot be obtained, and even in such a range, a sufficient eye relief with respect to the light rays from the periphery of the field-of-view cannot be maintained.
Moreover, in any of the above mentioned eyepiece optical systems, the change in the apparent visual angle upon the adjustment of the diopter has not been considered. The adjusting of the diopter is usually performed while a photographer is looking through the finder, and accordingly, is unpleasant for the photographer to observe the apparent visual angle being varied. In particular, when the diopter is suitably set for the photographer, if the apparent visual angle is narrowed, the feel of unpleasantness may be intensified.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an eyepiece optical system for a single lens reflex camera in which a 135-type film is used, and a penta mirror is provided as an erecting optical system; through which a diopter adjustment can be performed. Moreover, even when the diopter adjustment is being performed, change in the apparent visual angle is small. Furthermore, it is another object to provide a smaller sized eyepiece optical system in which a penta mirror is employed, and a sufficient eye relief with respect to the light rays from the periphery of the field-of-view can be maintained.
In order to achieve the above mentioned objects, there is provided an eyepiece optical system for a single lens reflex camera, which utilizes a penta mirror. The eyepiece optical system includes a negative first lens group, a positive second lens group, and a negative third lens group, in this order from the penta mirror; wherein the second lens group is a diopter adjustment lens group which is adjustable along the optical axis direction; and the following conditions are satisfied:
−0.8<f
1G
/f
e
<−0.2  (1)
0.2<f
2G
/f
e
<0.5  (2)
−15<f
3G
/f
e
<−1  (3)
wherein
f
1G
designates the focal length of the first lens group;
f
2G
designates the focal length of the second lens group;
f
3G
designates the focal length of the third lens group; and
f
e
designates the focal length of the entire eyepiece optical system when the diopter is −1.0.
The eyepiece optical system preferably satisfies the following condition:
0.02<d
3G
/f
e
<0.07  (4)
wherein
d
3G
designates the equivalent air thickness from the vertex of the most penta-mirror-side surface to the vertex of the most photographer's-eye-side surface in the third lens group.
In an eyepiece optical system according to the present invention, the first lens group can be constituted as a two-lens-element lens group which includes a positive lens element and a negative lens element, in this order from the penta mirror, and the third lens group can be constituted by a negative meniscus lens having a concave surface on the side of the photographer's eye. Further, the above eyepiece optical system preferably satisfies the following condition:
−15<f
3G
/f
e
<−5  (3′)
Alternatively, the first lens group can be constituted by a negative meniscus lens having a concave surface on the side of the penta mirror, and the third lens group can be constituted as a two-lens-element lens group which includes positive lens element and a negative lens element, in this order from the penta mirror. The eyepiece optical system referably satisfies the following condition:
−6<f
3G
/f
e
<−  (3″)
The present disclosure relates to subject matter contained in Japanese Patent Application No.10-278569 (filed on Sep. 30, 1998) which is expressly incorporated herein by reference in its entirety.


REFERENCES:
patent: 4389097 (1983-06-01), Ohishi
patent: 4664485 (1987-05-01), Momiyama et al.
patent: 5136427 (1992-08-01), Sugawara
patent: 5193030 (1993-03-01), Nozaki et al.
patent: 5313327 (1994-05-01), Ito et al.
patent: 59-148021 (1984-08-01), None
patent: 9-146019 (1997-06-01), None

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