AUTOMATIC FOCUSING SYSTEM

PURPOSE:To accomplish automatic focusing based on output from one or a few photodetectors. CONSTITUTION:This system is provided with a photoelectric conversion part 1, an image-formation part 2 which forms the image of an object on the conversion part 1, a focus shifting plate 3 which is constituted of a light transmissive material having specified thickness and which is allowed to get in and out of an optical path between the conversion part 1 and the image-formation part 2 to shift a position where the image of the object is formed, a focus divergence detection part 4 which detects the fluctuation amount E of the output from the conversion part 1 caused by allowing the plate 3 to get in and out of the optical path, and a focusing control part 5 which controls the focusing of the image-formation part 2 in a direction where the fluctuation amount E is reduced based on the fluctuation amount E. Then, the photoelectric conversion part 1, the image-formation part 2, and the focus shifting plate 3 are constituted of a member which makes infrared rays symmetric.