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.