1. A focus detecting system for detecting the focus condition of a constantly moving image in an optical system, comprising: a) means for detecting the sharpness of a plurality of sub-images formed along a plurality of sight lines by said optical system said detecting means including a plurality of detectors arranged in a plane tilted with respect to an image plane of said optical system and said detectors aligned with a direction of motion of the image formed by said optical system such that said sub-images are detected at a plurality of positions in the vicinity of best focus of the optical system; b) means for providing a time delay between said sub-images such that the sub images represent the same portion of a scene being imaged by said optical system; and c) means responsive to said sub-images for computing a position of best focus for said optical system.