RANGE FINDER FOR PASSIVE TYPE AUTOFOCUSSING DEVICE

A range finder for a passive type autofocussing device arranged so that light rays emitted from a scene to be photographed are picked up by three photosensors, output signals therefrom representing a luminance distribution of the scene are A/D converted, the zero-cross points of secondary differences of these digitized values are detected, zero-cross signals are compared with one of said three photosensors being selected as a reference and the zero-cross behaviors associated with the other two photosensors being successively shifted relative to said reference until a coincidence of the zero-cross behaviors associated with said three photosensors is detected, and a range to the scene is computed based on an amount that the zero-cross behaviors associated with said other two photosensors have been shifted until the coincidence is detected. Alternatively, said secondary differences of the digitized values are interpolated to detect the zero-cross points. Alternatively, the secondary difference of the luminance distribution is interpolated to compute and detect a zero-cross point and a range to the scene is found based on a coincidence of the zero-cross behaviors associated with the three photosensors.