Focus detecting optical system

Abstract

A focus detecting optical system comprises an objective lens for forming an image of an object, at least one condenser lens disposed behind a predetermined image plane of the objective lens for converging light passed through the objective lens, at least one pair of image re-forming lenses located behind the condenser lens for re-forming a pair of secondary images of the image formed by the objective lens, at least one pair of rows of light receiving elements arrayed in a single line behind and in alignment with the paired image re-forming lenses for detecting the paired secondary images, and a light deflecting member disposed between the image re-forming lenses and the element rows and separated from said image re-forming lenses by an air space for deflecting light fluxes forming the secondary images outwards in a direction of array of the element rows with respect to a center of the paired secondary images. By such a construction, a state of focus adjustment of the objective lens can be detected on the basis of a change of the position of the secondary images detected by the element rows.