Optical: systems and elements – Prism – With reflecting surface
Reexamination Certificate
1999-11-30
2001-06-26
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Prism
With reflecting surface
C359S362000, C359S431000, C359S836000
Reexamination Certificate
active
06252729
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a real image type finder optical system for use in a viewfinder of cameras. The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei 10-344339 (filed on Dec. 3, 1998), which is expressly incorporated herein by reference in its entirety.
2. Description of the Related Art
As shown in the plan view of
FIG. 12
, in a conventional real image type finder optical system for use in compact cameras and the like, light beams from an object are transmitted through an objective optical system
100
to be converged, reflected 90° laterally by a roof mirror
101
while being inverted upside down along the vertical direction that is perpendicular to the plane of
FIG. 12
, and forms an erected image of an object on the plane of a condenser lens
102
once. The light beams having formed the erected image of the object enter a penta prism
103
while being diverged. The light beams are reflected twice, each by 45°, within the plane including optical axes of the objective optical system
100
and the condenser lens
102
in the pentagonal prism
103
and thereafter exit the penta prism
103
in parallel to the optical axis of the objective optical system
100
. The light beams coming out of the penta prism
103
are transmitted through an eyepiece lens
104
to form the virtual image of the object which is observed by a photographer. Incidentally, the cover glass
105
in
FIG. 12
is a plane parallel plate fit into the camera casing.
Since its optical axis extending from the objective lens
100
to the eyepiece lens
104
(hereinafter, referred to as “finder optical axis”) is bent only within the same plane, a conventional real image type finder optical system having such configuration as described above may be small in vertical dimension as compared with ones using a Porro prism.
Now, for the purpose of making a diopter adjustment in conformity to the photographer's vision possible, it is desirable to design the eyepiece lens
104
so as to be movable along the direction of the finder optical axis. Therefore, a moving range &agr; of the eyepiece lens
104
must be provided behind the exit surface
103
a
of the penta prism
103
.
According to the conventional real image type finder optical system shown in
FIG. 12
, however, the entire finder optical path extending from the objective optical system
100
to the fore-end of the moving range &agr; of the eyepiece lens
104
(that is, the exit surface
103
a
of the penta prism
103
) lies before (above, in
FIG. 12
) the fore-end of the moving range &agr; of the eyepiece lens
104
(that is, the exit surface
103
a
of the penta prism
103
). This means that the moving range &agr; of the eyepiece lens
104
can not overlap with the finder optical path extending from the objective optical system
100
to the fore-end of the moving range &agr; of the eyepiece lens
104
(that is, the exit surface
103
a
of the penta prism
103
) in the front-to-back direction (the top-to-bottom direction in FIG.
12
). Consequently, conventional real image type finder optical systems cannot be constituted very compact in overall length along the front-to-back direction, and the cover glass
105
needs to be projected from the camera casing.
SUMMARY OF THE INVENTION
In view of the above-described problems in conventional real image type finder optical systems, it is an object of the present invention to provide a real image type finder optical system in which the moving range of the eyepiece lens overlaps with the optical path extending from the objective optical system to the fore-end of the moving range of the eyepiece lens in the front-to-back direction to reduce the overall length of the system along the front-to-back direction.
A real image type finder optical system according to a first aspect of the present invention contrived to attain the foregoing object has an objective optical system and an eyepiece lens arranged so that their optical axes are parallel to each other. In the real image type finder optical system, an optical axis extending from the objective optical system to the eyepiece lens is bent at least two times in parallel with a prescribed plane. This real image type finder optical system comprises a first prism which has a first face as an entrance surface confronting an exit plane of the objective optical system, a second face opposed aslope to the first face in order to obliquely bend the optical axis of the objective optical system toward the first face and a third face as an exit surface for transmitting the optical axis bent in order by the second face and the first face. The finder optical system further comprises a second prism which has a first face as an entrance surface for transmitting the optical axis transmitted through the first prism, a second face opposed aslope to the first face in order to obliquely bend the optical axis to an object side and toward the first face, a third face for further bending the optical axis bent in order by the second face and the first face so as to be substantially coaxial with that of the eyepiece lens and a fourth face as an exit surface for transmitting the optical axis bent by the third face.
In such constitution, the fourth face of the second prism may be located on an objective side of the eyepiece-side margin of the optical path extending from the first face to the second face of the second prism, in the front-to-back direction. Meanwhile, the moving range of the eyepiece lens is provided along the finder optical axis extending in the front-to-back direction. Therefore, the optical path from the objective optical system to the fore-end of the moving range of the eyepiece lens may overlap with the moving range of the eyepiece lens in the front-to-back direction. As a result, the real image type finder optical system according to the present invention can be reduced in overall length along the front-to-back direction.
A real image type finder optical system according to a second aspect of the present invention has an objective optical system and an eyepiece lens arranged so that their optical axes are parallel to each other. In the real image type finder optical system, an optical axis extending from the objective optical system to the eyepiece lens is bent at least two times in parallel with a prescribed plane. This real image type finder optical system comprises a first prism which has a first face as an entrance surface confronting an exit plane of the objective optical system, a second face opposed aslope to the first face in order to obliquely bend the optical axis of the objective optical system toward the first face and a third face as an exit surface for transmitting the optical axis bent in order by the second face and the first face. The finder optical system further comprises a second prism which has a first face as an entrance surface for transmitting the optical axis transmitted through the first prism, a second face opposed aslope to the first face in order to obliquely bend the optical axis to an object side and toward the first face and a third face as an exit surface for transmitting the optical axis bent in order by the second face and the first face. The finder optical system further comprises a reflecting member which has a reflective surface for bending the optical axis transmitted through the second prism so as to be substantially coaxial with that of the eyepiece lens.
In such constitution, the reflective surface of the reflecting member may be located on an objective side of the eyepiece-side margin of the optical path extending from the first face to the second face of the second prism, in the front-to-back direction. Meanwhile, the moving range of the eyepiece lens is provided along the finder optical axis extending in the front-to-back direction, or along the finder optical axis extending from the third face of the second prism toward the reflecting member. Therefore, the optical path from the objective optical system to the fore-end o
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Nguyen Thong
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