JPS6456407A - Focus detector - Google Patents

Abstract

PURPOSE:To precisely detect even an optical system including optical elements which are asymmetrical with respect to the division direction of a pupil by making the lengths of optical paths of the optical system for forming a pair of optical distributions of the image of an object different, putting the extension of the optical axis of an objective lens between the optical paths, in terms of the division direction of the pupil of the objective lens. CONSTITUTION:As for a light beam which is directed from a point deviated lower than the optical axis L on an intended image-formation surface 2 to transducer trains 6 and 7, a fine difference occurs between spaces Z1 and Z2 by receiving the effect of the residual aberration of the lens since the incident angles are different in secondary image-formation lenses 4 and 5. In order to correct the difference, one-sided surfaces of both secondary image-formation lenses are made flat and uniformly inclined in the alignment direction of the transducers. Thus, the precise detection can be attained without receiving the effect of the optical member which is rotational asymmetry with respect to the optical axis.