This invention relates to a system for automatic focus adjustment of an optical system particularly adapted for use in a photographic camera which utilizes non-linear resistance versus illumination response characteristics which are prominent in photoconductive cells such as cadmium sulfide and cadmium selenide cells. In one embodiment, a pair of photoconductive cells, one of which has a photoconductive surface having elongated side portions which contact the electrodes along the length of the elongated side portions and another cell which has an elongated photoconductive surface and shortened end portions and which contact the electrodes only at the shortened end portions, are arranged so that portions of an image-forming light beam entering through an objective of the camera are directed to the photoconductive surfaces on which similar real images of an object are formed. The objective includes at least one lens member movable for focusing associated with an automatic focus control mechanism operative in response to variation of an electrical output signal of the photoconductive cell assembly produced when the degree of sharpness of the image on each of the photoconductive surfaces is varied as the focussing member is moved for focussing purposes with respect to the photoconductive cell assembly.