| 摘要 |
PURPOSE:To detect a focus of the whole area even in case of a lens whose extension length is large, by placing the third photodetector at a position being separated from a position before and behind a prearranged image forming surface, and constituting so that optical path length of a luminous flux which is made incident to this photodetector is variable. CONSTITUTION:A luminous flux from a lens 2 is provided with desired optical path length by a half-mirror 91 and 92, and is made incident to a photodetector 111 and 112. At a position separated from a position of these photodetectors 111, 112, the third photodetector 113 is placed, and between a mirror 93 for leading the luminous flux to this photodetector 113 and the photodetector 113, an optical length expanding means consisting of, for instance, prisms 18, 19 is provided. Accordingly, an interval between the prisms 18 and 19 can be varied freely, by which a focus can be detected over the whole area even if the extension length of an optional lens is increased. |
