Zoom finder

Television – Camera – system and detail – Optics

Reexamination Certificate

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Details

C348S341000, C396S379000

Reexamination Certificate

active

06252628

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a zoom finder, and more particularly to a zoom finder which is able to prevent an image from shifting.
2. Description of Related Art
A conventional zoom finder has a finder optical system which is composed of an objective lens including two movable lenses, an erecting optical system, an eyepiece, etc. One end of both movable lenses is supported movably along the optical axis by a guide shaft, and the opposite end of both movable lenses engages with a rotation preventive member. The movable lenses are pressed by a spring so that cam followers provided at the movable lenses can be in contact with cam faces of a finder cam. Thus, by driving the finder cam, the two movable lenses are moved along the optical axis in a certain relation to thereby change a magnification of the zoom finder.
When the two movable lenses are moved by driving the finder cam, a frictional force is produced between the cam face of the finder cam and the cam follower of the movable lens. The frictional force acts on the cam follower so as to rotate the movable lens around the guide shaft. Although one end of the movable lens engages with the rotation preventive member, the movable lens is rotated a little by the frictional force since there is some play between them. When the driving directions of the finder cam are switched, the directions of the frictional force are also switched, and then the rotational directions of the movable lens are switched. Thus, the conventional zoom finder has a problem in that the image observed through the finder shifts whenever the driving directions of the finder cam are switched, as a result of the shifts of the movable lenses.
SUMMARY OF THE INVENTION
The present invention has been developed in view of the above-described circumstances, and has as its object the provision of a zoom finder which is able to easily prevent the image from shifting without increasing the number of parts.
To achieve the above-mentioned object, a zoom finder of the present invention comprises: first and second movable lenses composing a variable magnification optical system of the zoom finder, each of the first and second movable lenses being provided with a guide hole, an engaging part and a cam follower; a guide shaft for supporting the first and second movable lenses movably along an optical axis of the variable magnification optical system, the guide shaft being arranged parallel to the optical axis and being inserted into the guide holes of the first and second movable lenses; a rotation preventive member for preventing the first and second movable lenses from rotating around the guide shaft by engaging with the engaging parts of the first and second movable lenses, the rotation preventive member being arranged parallel to the optical axis; a finder cam for moving the first and second movable lenses along the optical axis by moving first and second cam faces which are provided at the finder cam and are respectively in contact with the cam followers of the first and second movable lenses; a pressing member for pressing the cam followers of the first and second movable lenses against the first and second cam faces of the finder cam; and the zoom finder is characterized in that at least one of the first and second cam faces of the finder cam inclines, over the whole area for use, to one side with respect to a direction in which a pressing force of the pressing member is applied to the cam follower of at least one of the first and second movable lenses which is in contact with the at least one of the first and second cam faces, so that the engaging part of the at least one of the first and second movable lenses can always be in contact with the rotation preventive member by operation of a component of the pressing force of the pressing member in a direction of a tangent line of the at least one of the first and second cam faces at a point where the at least one of the first and second cam faces is in contact with the cam follower of the at least one of the first and second movable lenses.
According to the present invention, the cam face of the finder cam inclines, over the whole area for use, to one side with respect to the direction of the pressing force of the pressing member applied to the cam follower which is in contact with the cam face. Thereby, the component of the pressing force in the direction of the tangent line of the cam face at the point where the cam face is in contact with the cam follower, which component acts on the cam follower so as to rotate the movable lens around the guide shaft, is always directed to one side. By making the component of the pressing force larger than the frictional force which is produced between the cam face and the cam follower when the cam face moves, a rotational force which is always directed to one side can be applied to the movable lens, and then the engaging part of the movable lens can always be in contact with a regulating face of the rotation preventive member. Consequently, the movable lenses can be prevented from rotating, and thus the image can be prevented from shifting.


REFERENCES:
patent: 5678090 (1997-10-01), Nishimura et al.
patent: 5778262 (1998-07-01), Kobayashi et al.
patent: 5848302 (1998-12-01), Machida
patent: 5884106 (1999-03-01), Manabe

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