Focus detecting system using a beam splitter to form an image at nominal best focus, slightly in front of best focus and slightly behind best focus

摘要

A focus detecting system for detecting focus state of an image forming optical system relative to an object, includes a beam splitter assembly for forming at least three coplanar two-dimensional images of a scene, each image having a different focus setting. An area array image sensor senses the images at the image forming plane. Signal processing electronics determine the sharpness of the three images and calculates an optimum focus setting for the optical system by deterring the variances of the three images and calculating a position of maximum variance corresponding to a best focus setting of the optical system.