FOCUS DETECTING METHOD

PURPOSE:To exactly detect a focus with high accuracy, by using an image pickup element tightly provided on the reverse side of a diffraction grating, and detecting a focused state by means of contrast measurement. CONSTITUTION:An image of an object is formed on the surface of a diffraction grating 2 by an objective lens 1. An image pickup element 3 is provided tightly on the reverse side of the diffraction grating 2. In case of dissidence of a focus, a multiimage is formed, therefore, contrast of the image drops and as the image approaches coincidence of a focus, the contrast rises and becomes maximum in a focused state. Accordingly, the focus is detected by measuring the contrast of an image in case when the focus is moving, by the image pickup element 3 and a detecting circuit 5.