Camera having interchangeable objective lens

An optical filter, such as a low-pass filter, which is disposed between a photo-taking lens mounted on a mount part of the camera body having an image sensor disposed on an image forming plane and a main mirror disposed within the camera body, is held in position jointly by a holding member fixed to the lower end of a front aperture part of the camera body and an elastic member fixed to the rear side of the holding member with a resilient force applied to the optical filter. In mounting the photo-taking lens on the camera body, when an end part of the photo-taking lens comes to touch the optical filter, the optical filter is allowed to temporarily move to escape from the photo-taking lens and to come back to its original position upon completion of the lens mounting operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a single-lens reflex type camera having a 
solid-state image sensor and arranged to be capable of recording 
information by using the solid-state image sensor and to permit use of an 
interchangeable photo-taking lens. 
2. Description of Related Art 
A number of cameras of the single-lens reflex type having a light flux 
obtained through a photo-taking lens used both by a viewfinder optical 
system for observing a photo-taking object and by an image forming system 
for taking a picture of the object have been developed to use a 
solid-state image sensor in place of a silver-halide film at an image 
recording part. However, if accessories used for the conventional 
single-lens reflex cameras, such as interchangeable lenses, external 
electronic flash devices, etc., are not usable for the cameras having the 
solid-state image sensors, it is greatly disadvantageous not only to the 
users of conventional single-lens reflex cameras but also to the providers 
of the cameras. 
Accordingly, a number of cameras have been variously proposed in which, to 
provide a camera having a solid-state image sensor without varying the 
system interchangeability of single-lens reflex cameras, the solid-state 
image sensor is placed at a focal image forming portion corresponding to 
the position of the silver-halide film on the base of the conventional 
silver-halide-film-using single-lens reflex camera. 
In the case of such a camera having a solid-state image sensor, a CCD image 
sensor or the like is used in general. However, such an image sensor has a 
spectral sensitivity characteristic which requires some correction. To 
meet this requirement, some optical filters are used in general. These 
filters include a low-pass filter which is used for avoiding a moire 
pattern resulting from a relation between the number of pixels of the 
image sensor and the high spatial frequency of the photo-taking object and 
for avoiding a spurious color obtained in the case of a color image 
sensor, and an infrared cut filter for cutting off an infrared component. 
As disclosed in Japanese Patent Publication No. HEI 4-435, the optical 
filter can be disposed in a space immediately before a shutter. 
However, such a disposition of the optical filter in that space brings 
about the following two problems. 
Firstly, unlike the conventional single-lens reflex camera, the camera of 
the kind having a solid-state image sensor requires no film replacing 
action and thus requires no back lid (back cover) arranged to be opened 
and closed for replacing a film. Therefore, if it is taken into 
consideration that the work of removing dust attached to the optical 
filter or replacing the optical filter with another optical filter is 
performed through a lens-mount opening part of the camera, the existence 
of a main mirror makes it very difficult to perform that working. 
Secondly, since a light flux coming from a photo-taking object is split by 
the main mirror into light fluxes which are directed respectively toward a 
viewfinder system and a photo-taking system and, then, only the light flux 
directed to the photo-taking system passes through the optical filter, a 
change in optical path length caused by the optical filter takes place 
only in the photo-taking system. Hence, the optical path length of the 
viewfinder system must be made to be equal to that of the photo-taking 
system either by providing the viewfinder system also with an optical 
filter or by modifying the design of optical path length of the viewfinder 
or that of the total optical path length of the photo-taking system. 
However, such a modification not only necessitates redesign work but also 
results in an increase in size of the camera. 
To solve the above-stated two problems, it has been considered to change 
the installing position of the optical filter to a position immediately 
before the main mirror, i.e., immediately behind the photo-taking lens. 
Such a change in the installing position of the optical filter solves the 
above first problem to facilitate the above-stated optical filter cleaning 
and replacing work, because the optical filter is located immediately 
behind the lens-mount opening part. 
As for the above second problem, since the object light flux is split by 
the main mirror to the viewfinder system and the photo-taking system after 
passing through the optical filter, the change in optical path length is 
equally caused by the optical filter both in the viewfinder system and the 
photo-taking system. Therefore, a change in the focal image forming 
position resulting from an increase of the optical path length due to the 
optical filter can be corrected by just changing the thickness of the 
mount. 
FIG. 7 shows by way of example the arrangement of the conventional 
single-lens reflex type camera having a solid-state image sensor in which 
an optical filter is installed immediately before a main mirror. Referring 
to FIG. 7, a bayonet mount part 2 is provided at the front surface of a 
camera body 1. A photo-taking lens (not shown) is detachably mounted on 
the mount part 2. An optical filter 3 which is a low-pass filter or an 
infrared cut filter is disposed between the mount part 2 and the main 
mirror 4. When the user of the camera observes an object image, an object 
light flux coming through the photo-taking lens passes through the optical 
filter, so that a spectral sensitivity characteristic of the object light 
flux is corrected there. After the correction, a part of the object light 
flux is reflected upward almost at a right angle by the main mirror 4, 
which is a half-mirror. The reflected object light flux passes through a 
focusing screen 5 and a condenser lens 6 to be converted into an erecting 
image by a vertical and lateral inverting action of an erecting image 
forming element 7. The light flux of the erecting image thus obtained is 
led to the user through an eyepiece lens group 8. In addition, another 
part of the object light flux passes through the main mirror 4 and is then 
totally reflected by a total-reflection submirror 9. The light flux 
reflected by the submirror 9 is led to an automatic focusing sensor 13 
through an automatic focusing optical system (10, 11 and 12) so as to 
detect a focusing state of the photo-taking lens. 
The optical filter 3 is interposed in between a support member 14 and a 
fixing plate 15 to be fixed in position perpendicularly to an optical axis 
and not to become aslant. The support member 14 is mounted, integrally 
with the optical filter 3, on the camera body 1 with a screw (not shown). 
In taking a shot at an object, the main mirror 4 and the submirror 9 
required for the above-stated observation and focus detection are 
beforehand retracted from the path of the object light flux between an 
objective lens group of the photo-taking lens and a CCD image sensor 16 
serving as a focal plane. 
Next, a photo-taking light flux having passed through the objective lens 
group of the photo-taking lens passes through the optical filter 3 to have 
a spectral sensitivity characteristic thereof corrected there. When a 
fully-open state of shutter leading blades (front curtain) 17a is 
obtained, an image of the photo-taking object is formed on the CCD image 
sensor 16, which is mounted from the side of a back lid 18 in a focal 
image forming position that is about the same as the position of a 
silver-halide film in a single-lens reflex camera. After the CCD image 
sensor is exposed to the light of the object image to accumulate it for a 
predetermined period of time, the light flux of the photo-taking object 
image is cut off by a closing action of shutter trailing blades (rear 
curtain) 17b. After that, information on the image thus picked up is 
transferred to be recorded as an image signal on a storage device disposed 
either within the camera body or outside of the camera body. 
The single-lens reflex type camera arranged to have the optical filter 
mounted immediately before the main mirror as described above, however, 
has the following problem. Since the main mirror 4 is arranged to be 
retracted upward at the time of an exposure, the size of a flange back 
part must be increased for the purpose of preventing the main mirror 4 
from interfering with the optical filter 3. This increase causes an 
increase in size of the camera. 
This problem may be solved by arranging the main mirror 4 in a smaller size 
to prevent the main mirror 4 from interfering with the optical filter 3. 
However, in mounting a photo-taking lens on the above-stated camera having 
a solid-state image sensor, the base of which is the conventional 
single-lens reflex camera using a silver-halide film, an end part of the 
photo-taking lens tends to damage, or might break, the surface of the 
optical filter if the end part of the photo-taking lens comes to touch the 
optical filter. This problem still remains to be solved. 
BRIEF SUMMARY OF THE INVENTION 
The invention is directed to the solution of the above-stated problem. It 
is, therefore, an object of the invention to provide a camera having a 
solid-state image sensor, in which an optical filter is prevented from 
being damaged by a mounting or demounting operation on a photo-taking lens 
and is easy for maintenance work. 
To attain the above object, in accordance with an aspect of the invention, 
there is provided a camera having a solid-state image sensor, in which an 
optical filter is disposed between a photo-taking lens and a main mirror 
and the optical filter is held by a support member which supports the 
optical filter and an elastic member which pushes the optical filter 
against the support member. 
The above and other objects and features of the invention will become 
apparent from the following detailed description of a preferred embodiment 
thereof taken in connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Hereinafter, a preferred embodiment of the invention will be described in 
detail with reference to the drawings. 
FIG. 1 is a vertical section view of a camera having a solid-state image 
sensor according to the embodiment of the invention. FIG. 2 is an exploded 
perspective view of an optical filter unit of the camera. FIG. 3 is a 
front view of the camera with a photo-taking lens demounted. FIG. 4 is an 
exploded perspective view showing the manner of fixing the optical filter 
unit to the camera body. 
The structural arrangement of the camera having a solid-state image sensor 
shown in FIG. 1 is the same as that of the conventional camera shown in 
FIG. 7, except for the support structure for the optical filter unit. 
Therefore, the parts of the camera shown in FIG. 1 which are the same as 
those shown in FIG. 7 are denoted by the same reference numerals as in 
FIG. 7, and the details of them are omitted from the following 
description. 
Referring to FIGS. 1 and 2, a support member 21 is arranged to support an 
optical filter 3. The support member 21 is provided with an aperture part 
21a for restricting the amount of an object light flux coming through the 
photo-taking lens, and a picking part 21b which is formed along the upper 
edge of the aperture part 21a to serve as a knob in mounting and removing 
the optical filter unit on and from the camera body 1. Further, the 
support member 21 is provided with a pair of projections 21c and an 
L-shaped bent part 21d, which are formed respectively on the upper and 
lower edges of the support member 21. Furthermore, a pair of pin holes 21e 
for receiving caulking pins 23 are formed in the edge part of each of two 
sides of the support member 21, and the L-shaped bent part 21d is provided 
with a hole 21f for a stop screw 25 which will be described later herein. 
Referring to FIGS. 2 and 4, a pair of elastic members 22 are arranged to 
push the optical filter 3 against the support member 21 so as to ensure 
that the optical filter 3 never moves nor comes off when the camera is 
moved. Each of the elastic members 22 is composed of a restricting part 
22a and an elastic part 22b. The restricting part 22a has two faces. One 
face of the restricting part 22a defines the position of the optical 
filter 3 in the direction of "x" on a plane orthogonally intersecting the 
direction of an optical axis, while the other face thereof slants outward 
at the upper end of the restricting part 22a. The restricting part 22a is 
thus arranged to guide the optical filter 3 when the optical filter 3 is 
inserted. The elastic part 22b exerts a resilient force to push the 
optical filter 3 against the support member 21. The caulking pins 23 are 
inserted into the pin holes 21e of the support member 21 to have the 
elastic member 22 secured there by caulking. 
Referring to FIGS. 3 and 4, a shield member 24 is fixedly secured to the 
upper edge side of a front aperture part of the camera body 1. The shield 
member 24 has a pair of open parts 24a formed in its lower edge part to 
insert therein the projections 21c of the support member 21. The stop 
screw 25 is arranged to secure the support member 21 to the lower edge 
side of the front aperture part of the camera body 1 through the hole part 
21f provided in the support member 21. 
The optical filter unit having the above structure is assembled and fixed 
to the camera body 1, as described below with reference to FIGS. 2 and 4. 
With the elastic member 22 in a state of having been fixed to the support 
member 21 with the caulking pins 23, the optical filter 3 is first slid 
from above in between the support member 21 and the elastic parts 22b of 
the elastic member 22. The optical filter 3 is thus inserted until the 
lower end face of the optical filter 3 abuts on the bent part 21d of the 
support member 21. Thus, the optical filter unit is assembled in the above 
manner. 
Next, the projections 21c provided at the upper part of the support member 
21 are inserted into the pair of open parts 24a of the shield member 24 
fixed to the front aperture side of the camera body 1. Then, the optical 
filter unit is fixedly secured to the camera body 1 with the stop screw 25 
inserted into the lower hole part 21f of the support member 21. 
With the optical filter unit thus fixed to the camera body 1, in mounting a 
photo-taking lens 26 on the camera body 1 at the bayonet mount part 2, the 
optical filter unit acts as follows, as shown in FIGS. 5 and 6. 
As shown in FIG. 5, in mounting the photo-taking lens 26 on the bayonet 
mount part 2, an end part 26a of the photo-taking lens 26 comes to touch 
the surface of the optical filter 3. Therefore, there arises some force of 
pushing the optical filter 3 with the photo-taking lens 26. When this 
pushing force comes to exceed a certain amount of force, the resilient 
force of the elastic part 22b of the elastic member 22, which keeps the 
optical filter 3 in contact with the support member 21, yields to the 
pushing force, so that the optical filter 3 slants as shown in FIG. 5. 
Accordingly, since the optical filter 3 moves in the direction of escaping 
from the photo-taking lens 26, the optical filter 3 is effectively saved 
from receiving any force that exceeds a predetermined value. 
The optical filter 3 thus can be prevented from being damaged or broken by 
the photo-taking lens 26. 
In this instance, the picking part 21b of the support member 21 is located 
closer to the photo-taking lens 26 than the aperture part 21a in the 
direction of the optical axis and serves to prevent the end part 26a of 
the photo-taking lens 26 from touching the upper half of the surface of 
the optical filter 3 which extends perpendicular to the optical axis. 
Next, the end part 26a of the photo-taking lens 26 no longer touches the 
optical filter 3 after completion of mounting the photo-taking lens on the 
bayonet mount part 2. As a result, the optical filter 3 is pushed only by 
the resilient force of the elastic part 22b of the elastic member 22 and 
is thus allowed to resume its state of abutting on the support member 21, 
as shown in FIG. 6. 
As described above, in accordance with the invention, in a camera of the 
kind having a solid-state image sensor and an interchangeable photo-taking 
lens with an optical filter disposed between the photo-taking lens and the 
main mirror, a fixing member for the optical filter is composed of a 
support member which supports the optical filter and an elastic member 
which causes the optical filter to abut on the support member with its 
elastic part. This arrangement allows the optical filter to move in the 
direction of escaping from the photo-taking lens when the photo-taking 
lens comes to touch the optical filter in mounting the photo-taking lens 
on the camera body. Therefore, the photo-taking lens can be mounted on the 
camera body without damaging or breaking the optical filter. Besides, the 
arrangement facilitates maintenance work on the optical filter. 
Further, according to the arrangement of the embodiment disclosed, the 
elastic member causes the optical filter to abut on the support member by 
exerting its resilient force on the side of the second surface of the 
optical filter located opposite to its first surface which faces the end 
part of the photo-taking lens in mounting the photo-taking lens. That 
arrangement ensures that the photo-taking lens can be mounted and 
demounted without damaging the optical filter, and the optical filter can 
be promptly set in its predetermined position after the photo-taking lens 
is mounted. 
Further, according to the invention, the optical filter is composed of a 
low-pass filter or an infrared cut filter. Therefore, the use of the 
optical filter enables the camera having a solid-state image sensor to 
take pictures with an excellent picture quality.