PURPOSE:To enable a stepless detection of a distance to a subject and to simplify a circuit constitution, by a method wherein a photopotention type semicondutor position detecting element is employed as a light-receiving element. CONSTITUTION:A light, emitted from a light-emitting element 1 mounted on a camera, which subjects 7-7'' reflects, is image-formed on a photopotention type light-receiving element 2 via a lens 4. The position of the spot on the light-receiving element 2 is determined corresponding to distances to the subjects, and a current corresponding to the distance is outputted from electrodes 2a and 2b. Said current is amplified by means of operational amplifiers 3a and 3b to obtain currents 11 and 12. The sum 3c and the sum 3d of the current 11 and 12 are found, and a ratio 3e of the sum 3c to 3d is amplified to obtain a position detecting signal VL. This permits the stepless detection of the distances to the subjects 7-7'' and the simplification of the constitution of a circuit 3.
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