FOCUS DETECTING METHOD

PURPOSE:To achieve focus detection with simple constitution by arranging adjacent photodetecting elements on the reverse surface of a diffraction grating so that their boundary line crosses the parallel direction of the grating, and then detecting the existence of a diffraction image. CONSTITUTION:Light from a spot light source 3 emitting light at constant cycles is passed through an anamorphic lens 4, a slit 5 and a lens 6 to strike a body, and an image of spot light is formed on a diffraction grating 7 through an objective lens 1. On the reverse surface of the diffraction grating 7, a photodetecting element 8 consisting of two photoelectric elements 8A and 8B whose boundary line cross the parallel direction of the grating of the diffraction grating is arranged in close contact. The image of the spot light is formed where its boundary is nearly coincident with the boundary line of the photoelectric elements 8A and 8B in an in-focus state. Therefore, the image is in focus when the maximum difference in output between the photoelectric elements 8A and 8B is obtained.