Abstract: In a focus detection system in which light from an object is incident on at least two photosensor arrays via an optical system to form two images on the photosensor arrays, the relative displacement between the two images is determined in order to detect whether the optical system is in focus or not. A correction term which is formed by f(i.sub.0 +2) and f(i.sub.0 +1) or f(i.sub.0 -2) and f(i.sub.0 -1) is added to f(i.sub.0-1)-f(i.sub.0) or f(i.sub.0 +1)-f(i.sub.0), so that the amount of image deviation between the two images is obtained. Here i.sub.0 is the value of i when a function f(i) is minimized, the function f(i) expressing the inconsistency between the two images, corresponding to an amount of relative displacement i.times.p between the two images obtained discretely. The symbol p represents the pitch of the at least two photosensor arrays.