Focus detection optical system in a single lens reflex camera

Disclosed is a focus detection optical system of a single lens reflex camera wherein a quick return mirror directs a part of luminous flux incident through a photographic lens system towards an optical sensor unit to detect the focusing state of the photographic lens system. The quick return mirror comprises a main mirror part which transmits said part of luminous flux while reflects the other part of luminous flux, and a sub-mirror part attached to the main mirror part so as to direct the part of luminous flux having transmitted the main mirror part toward the optical sensor unit. The main mirror part is provided with an infra-red cut coat which is formed on at least partial area of the light output side surface of the main mirror part which faces the sub-mirror part.

BACKGROUND OF THE INVENTION 
The present invention relates to a focus detection optical system in a 
single lens reflex camera, and more particularly to a quick return mirror 
for distributing a luminous flux incident from a photographic lens system 
to a finder optical system and to a focus detection sensor module. 
Heretofore, a focus detection sensor module has been employed in a single 
lens reflex camera having an auto-focus function. The single lens reflex 
camera is equipped with a quick return mirror which normally guides a 
luminous flux incident through a photographic lens system to a finder 
optical system except when exposure is executed. 
The focus detection sensor module is usually disposed at the bottom portion 
of a camera, and reflects downwardly a luminous flux transmitted through 
the central area of a main mirror part of the quick return mirror by means 
of a sub-mirror part so that the luminous flux is incident upon the sensor 
module. 
The focus detection sensor module has conventionally been constituted with 
fixing a visible field mask plate having light transmitting openings, a 
condenser lens, a mirror, an infra-red cut filter, a separator lens and a 
CCD (Charge Coupled Device) sensor unit onto a module main body so as to 
be arranged in this order from the side where light is incident. The 
infra-red cut filter may sometimes be interposed between the visible field 
mask plate and the condenser lens. 
However, the above described conventional focus detection sensor module is 
constituted with so many number of components as to require a large number 
of assembling processes as well as a large occupation space. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
structure of a quick return mirror which enables to reduce the number of 
required components in a focus detection sensor module. 
For the above purpose, in accordance with the present invention, an 
infra-red cut coat is formed on a sub-mirror side surface of a quick 
return mirror. More particularly, on a light incident side surface of a 
main mirror part, a half mirror coat is formed for allowing a part of 
incident light to pass therethrough while an infra-red cut coat is 
correspondingly formed on the opposite surface of the main mirror part to 
cut off the infra-red component of the light outputting from the main 
mirror part. 
In case the half mirror coat is formed only on the central area of the 
light incident side surface of the main mirror part, the infra-red cut 
coat is formed on the corresponding area on the opposite side surface of 
the main mirror part. Both coats may, however, be formed on the whole 
areas of both surfaces of the main mirror part.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A focus detection optical system in a single lens reflex camera embodying 
the invention will be described below in detail by referring the 
accompanying drawings. 
FIG. 1 is a perspective view showing a part of an optical system of a 
single lens reflex camera, and FIG. 2 is an enlarged side view of a quick 
return mirror employed in the optical system shown in FIG. 1. 
A luminous flux transmitted through a photographic lens system (not shown) 
to the left-hand side in FIG. 1 is incident upon a quick return mirror 10 
which is arranged in an optical path of the incident luminous flux. The 
quick return mirror 10 comprises a main mirror part 11 which reflects the 
incident luminous flux upwardly in FIG. 1, and a sub-mirror part 12. 
The sub-mirror part 12 is attached to the central area of the back surface 
of the main mirror part 11 so as to reflect downwardly, in FIG. 1, a 
luminous flux transmitted through a half mirror section formed on the 
central area of the main mirror part 11. 
The quick return mirror 10 is retracted from the optical path of the 
incident light only at the moment of exposure with a shutter release, and 
is normally situated in the optical path, as illustrated in FIG. 1. The 
luminous flux reflected by the main mirror part 11 is guided to a finder 
optical system 20. The finder optical system 20 comprises a focusing 
screen 21, a pentaprism 22, and an eyepiece lens system 23. On the other 
hand, the luminous flux reflected by the sub-mirror part 12 is incident 
upon a focus detection sensor module 30 which is disposed at the bottom 
portion of a camera body (not shown). 
The focus detection sensor module 30 is constituted by a visible field mask 
plate 31, a module main body 33, a CCD sensor unit 37 and other components 
housed in the module main body 33. 
As illustrated in FIG. 2, a half mirror coat C1 is formed on the central 
area of the surface 11a, which faces a photographic lens system (not 
shown), of the main mirror part 11. The surface 11a of the main mirror 
part 11 is constituted as a mirror surface having the light reflection 
ratio of about 100% except the central area thereof having the half mirror 
coat C1, through which a part of luminous flux is transmitted towards the 
sub-mirror part 12. 
On the other hand, on the central area of the other surface 11b, which 
faces the sub-mirror part 12, of the main mirror part 11, an infra-red cut 
coat C2 is formed. 
As the half mirror coat C1 is formed on the surface 11a, it is very 
difficult to design the central area of the surface 11a to also have an 
infra-red cut function. That is, it becomes necessary to design the coat 
C1 to have such a characteristic as to function as a half mirror for a 
visible light component and at the same time to function as an infra-red 
cut coat against an infra-red light component, which is not easy. Thus, in 
this embodiment, the infra-red cut coat C2 is formed on the other surface 
11b of the main mirror part 11. 
With thus constructed quick return mirror 10, the infra-red light component 
of the luminous flux incident through a photographic lens system can be 
cut off when the luminous flux passes through the main mirror part 11 of 
the quick return mirror 10. Accordingly, it is unnecessary to provide an 
infra-red cut filter in the focus detection sensor module 30, upon which 
the luminous flux deflected by the sub-mirror part 11b is incident. 
Further, as the infra-red cut coat C2 is positioned out of image forming 
area, even if dust or the like is adhered thereonto, it does not affect 
images captured by the CCD sensor unit 37. 
In the meantime, in the above embodiment, the half mirror coat C1 and the 
infra-red cut coat C2 are partly formed on the surfaces 11a and 11b of the 
main mirror part 11, which may, however, be formed on the whole areas of 
the surfaces 11a and 11b, as illustrated in FIG. 3. 
The present disclosure relates to a subject matter contained in Japanese 
Patent Application No. Hei 09-108254, filed on Apr. 10, 1997, which is 
expressly incorporated herein by reference in its entirety.