Patent US 4,333,716

Disclosed is a focus detecting system in which first and second signals corresponding to image forming states at first and second positions each almost at the same distance before and behind a predetermined focal plane of an image forming optical system are obtained. A third signal corresponding to an image forming state at a third position different from the first and the second positions is also obtained. The in-focus state of the image formed by the image forming optical system is detected on the basis of a first comparison signal obtained by comparing the first and the second signals with each other, a second comparison signal obtained by comparing the first and the third signals with each other and a third comparison signal obtained by comparing the second and the third signals with each other.