FOCUS DETECTING METHOD

PURPOSE:To obtain an inexpensive device which makes it possible to detect the coincidence between an imaging surface for an image and a diffraction grating, by detecting the fact that the ratio of outputs of two photodetecting elements, arranged at optically conjugate position, reaches a prescribed value. CONSTITUTION:On a diffraction grating 2, the image of a body to be measured is formed through an objective lens, and a beam splitter S is arranged behind the diffraction grating 2 to split light from the diffraction grating 2 into the 1st and 2nd light beams L1 and L2; and some of the image of the object on the diffraction grating 2 is formed on photodetecting elements 4 and 5 through imaging lenses 6 and 7 on the optical paths of the respective light beams. Filters 8 and 9 transmitting light beams having different spectral components are provided to form images of different spectral components on those photodetecting elements; for example, the filter 3 transmits only red light, and the filter 9 only blue light. When a diffracted image of red light and that of blue light are superposed mutually, namely, when the diffracted images disappear, an in-focus state is obtained.