TTL FOCUS DETECTING OPTICAL SYSTEM

PURPOSE:To constitute the above optical system in such a manner that the focus detection by the wavelength region of the same region as the photosensitive wavelength region of an image pickup element is executed by subjecting a mirror for guiding a luminous flux for focus detection to a distance measuring element to prescribed coating. CONSTITUTION:The mirror 3 or 4 for dividing a part of the luminous flux for image pickup and guiding the divided flux to the distance measuring element 6 provided separately from an imaging plane 2 of a lens 1 for image pickup is subjected to such coating which limits the wavelength region of the reflected luminous flux. The wavelength region limited by the coated reflecting surface is preferably matched with the wavelength region where the photosensitive element disposed on the plane 2 of the lens 1 has sensitivity. The coincidence of the wavelength region to be photographed by the image pickup and the wavelength region of the luminous flux detected by the distance measuring element is thereby attained and, therefore, good coincidence is admitted in the imaging stage of the image pickup lens and the focus detecting optical system.