in the arrangement disclosed , a photoconductive cell has electrical characteristics which depend upon its optical exposure . an optical system focuses an image upon the cell . an electrical system utilizes the output of the cell on the basis of the light upon the cell . the cell includes a multiplicity of photoconductive portions forming at least one row transverse to the direction of image intensity changes so that different portion sense different intensities . the electrical means are connected to a plurality of the portions . the portions may be formed into an elongated integral strip whose direction changes and whose edges are insulated from each other . alternately the portions may be connected electrically to form a continuous electrical strip . the strip may have a spiral shape . in one embodiment the portions are connected together to form an and or or gate . two identical cells may straddle a desired image plane and be subject to the same image . suitable means respond to the differences between the cells to drive a motor which adjusts an image - forming objective .