Focus detection optical system in a single lens reflex camera

摘要

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.