Photography – With viewfinder – With prism
Reexamination Certificate
1999-12-30
2001-11-06
Mahoney, Christopher E. (Department: 2851)
Photography
With viewfinder
With prism
C396S386000
Reexamination Certificate
active
06314246
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a viewfinder optical system which can be advantageously used for an SLR camera (e.g., an SLR camera using silver-halide film or a digital SLR camera) in which the distance between the image plane (e.g., the sensitive surface of an image pick-up device or a silver-halide film) and the back face of the camera has to be relatively long.
2. Description of the Related Art
SLR cameras are generally provided with a viewfinder optical system through which the object image formed on a focusing glass (i.e., a focusing screen) positioned above the mirror box is viewed as an erect image at the eye of the user (photographer). In most cases, the viewfinder optical system is of a type which includes a pentagonal roof prism and an eyepiece positioned behind the exit surface of the pentagonal roof prism. In this type of viewfinder optical system, an optical axis of the viewfinder optical system which extends through, and perpendicularly to, the approximate center of the focusing glass, is bent rearward via the pentagonal roof prism so as to extend parallel to the optical axis of the photographic lens to enter the eyepiece. The object image formed on the focusing glass is magnified by the eyepiece to be viewed by the user.
Due to the recent popularization of digital cameras, there has been an attempt to combine the advantage of an image processing function, which is unique to digital cameras, with the advantage of the optical system being free from parallax error, which is unique to SLR cameras, by combining the structure of a digital camera with the structure of an SLR camera. For example, there has been an attempt to replace the back lid of a conventional SLR camera with a back lid which has a CCD image sensor, an electric circuit and a data recording medium (e.g., a flash memory card or a floppy disk drive) provided therein, so that the conventional SLR camera can be used as an SLR digital camera. Furthermore, there has been an attempt to design the body of an SLR camera to be used exclusively as a digital camera body in which a CCD image sensor is fixed at a position at which sensitive film was formerly positioned in a conventional SLR camera.
FIG. 2
is a schematic illustration showing the arrangement of fundamental elements of a typical SLR digital camera. As shown in
FIG. 2
, a typical SLR digital camera is provided, behind a mirror box
12
, with a focal-plane shutter unit
13
, a CCD image sensor
14
and a recording medium
15
which are arranged in this order from the side of the mirror box
12
, which accommodates a quick-return mirror
11
therein. The SLR digital camera is provided above the mirror box
12
with a focusing glass
16
and a pentagonal roof prism
17
which are arranged in this order from the side of the mirror box
12
. The SLR digital camera is further provided behind the exit surface
17
a
of the pentagonal roof prism
17
with an eyepiece
18
. In the SLR digital camera having such a structure, the distance between the image plane (i.e., the sensitive surface of the CCD image sensor
14
), on which the object image is formed through the photographic lens
19
, and a back face C of the SLR camera is much longer than that of a typical SLR camera using silver-halide film.
The distance between the image plane and the back face of a multi-functional SLR camera using silver-halide film produced in recent years is also longer than that of a conventional simple SLR camera because such a multi-functional SLR camera often have various electric circuits, mechanisms and/or specially-prepared accessories provided behind the film plane.
In an SLR digital camera in which the distance between the sensitive surface of the CCD image sensor and the back face of the camera is long, a problem arises with the eyepiece of the camera. This problem will be hereinafter discussed.
In
FIG. 2
, in the case where the eyepiece
18
is arranged behind the pentagonal roof prism
17
at a position “A” shown by a dotted line, namely, wherein the eyepiece
18
is arranged at a position substantially identical to the position at which the eyepiece of a conventional SLR camera using silver-halide film is arranged, part of the camera body in which the CCD image sensor
14
and the recording medium
15
are accommodated bulges rearward by a considerable amount. Accordingly, when the user aligns his/her eye with the eyepiece, the camera body tends to interfere, especially around the cheek area. As a result, the user cannot look into the eyepiece
18
with his or her eye positioned closely to the eyepiece
18
, which makes it difficult for the user to view through the eyepiece
18
. To eliminate this problem, the viewfinder optical system can be designed so that the exit pupil of the eyepiece
18
is formed at a position on a plane, which includes the back face of the camera, by establishing a long eye relief of the eyepiece
18
(distance between the rear surface of the eyepiece
18
and the eye point thereof). However, according to such a design, both the lens diameter of the eyepiece
18
and the size of the pentagonal roof prism
17
must be increased, which makes the camera bulky (i.e., the physical size thereof is increased).
In order to eliminate the aforementioned problem without increasing either the lens diameter of the eyepiece
18
or the size of the pentagonal roof prism
17
, if the eyepiece
18
is fixed at a position “B” shown by a solid line in
FIG. 2
, the focal length of the eyepiece
18
must be made longer than that in the case where the eyepiece
18
is fixed at the position “A” (i.e., the focal length of the eyepiece
18
must be extended by a length corresponding to the distance between the positions A and B). As a result, the magnification of finder decreases to thereby arise a problem of decreasing the apparent visual angle relative to the subject.
SUMMARY OF THE INVENTION
The present invention has been devised so as to overcome the aforementioned problems which occur in SLR cameras in which the distance between the image plane, on which the object image is formed through the photographic lens, and the back face of the camera is long. The primary object of the present invention is to provide a viewfinder optical system which can be advantageously used for an SLR camera and which makes it possible to provide the eyepiece in the close vicinity of the back face of the camera without making the focal length of the eyepiece too long, to thereby provide a compact viewfinder having a high finder magnification that can be easily viewed via the eyepiece thereof.
To achieve the object mentioned above, according to an aspect of the present invention, a viewfinder optical system of a single-lens reflex camera is provided, having a photographic optical system and an image plane positioned on an optical axis of the photographic optical system, the camera photographing an object image formed on the image plane through the photographic optical system, the viewfinder optical system includes a first optical member positioned between the photographic optical system and the image plane, the first optical member bending the optical axis of the photographic optical system to define a viewfinder optical axis of the viewfinder optical system; a second optical member which bends the viewfinder optical axis at least once to make the viewfinder optical axis extend parallel to the optical axis of the photographic optical system; a third optical member, having substantially no optical power, positioned on the viewfinder optical axis extending parallel to the optical axis of the photographic optical system, the third optical member being made of a transparent material having a refractive index higher than the refractive index of air; and an eyepiece positioned behind the third optical member on the viewfinder optical axis extending parallel to the optical axis of the photographic optical system.
Preferably, the following condition (1) is satisfied:
0.2<
d/fe<
0.6 (1);
wherein “d” represents the actua
Ito Takayuki
Ozaki Hiroyasu
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Mahoney Christopher E.
LandOfFree
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