Abstract: A camera with a focusable objective including an axially shiftable lens member has two ancillary projection systems continuously or intermittently intercepting incident light rays downstream of that lens member and deflecting them onto respective inputs of a photoelectric comparator which emits a coincidence signal whenever these light rays project substantially identical images of an object to be photographed or filmed. One of the projection systems may include a periodically movable element which varies the relative orientation of the two beams constituted by the deflected light rays so that different angles of incidence give rise to a coincidence signal in different parts of a sweep cycle; in one position of this element, in which the incident rays will have a predetermined orientation (e.g. parallel to the objective axis) with proper focusing, a position sensor generates a correlation signal.

Abstract: A camera with a focusable objective has two secondary projection systems, at least one of which is continuously adjustable, working into a photoelectric comparator which emits a coincidence signal whenever the two projection systems are trained upon the same object along the optical axis of the main objective. A position sensor, coacting with a swingable mirror in the adjustable projection system, generates a correlation signal whenever the mirror position matches the setting of the main objective as determined by the axial position of a shiftable lens member thereof. With the swingable mirror set in oscillations to scan the object space in front of the main objective, the two signals are generated in the form of short pulses during each scanning sweep and are fed to a phase detector whose output may control a drive motor for the shiftable lens member to bring the main objective into correct focus.