Focusing optical system for single-lens reflex cameras

A focusing optical system for single-lens reflex camera comprising a photographic lens, a quick return mirror which is arranged for leading the light having passed through said photographing lens to a finder optical system and which has a section of incidence formed by removing a portion of the surface of said mirror, and an emerging section, and a detector element arranged in the vicinity of said emerging section, and said focusing optical system being adapted in such a manner that the surface of incidence of said section of incidence is arranged perpendicularly to the optical axis of the incident light, that the emerging surface of said emerging section is arranged perpendicularly to the emerging light, and that the light emitted from an object to be photographed and transmitted through said photographic lens is allowed to pass through said section of incidence into said quick return mirror, reflect repeatedly on the inside surfaces of said mirror, emerge from said emerging section and then fall on said detector element.