Focused point detecting optical system

摘要

The focussed point detecting optical system comprises an objective lens, a pair of reimaging lenses arranged on the side opposite to the objective lens at a location apart from the imaging plane of the objective lens, and a variable refractive index optical element arranged in the vicinity of said imaging plane so as to have, as a plane of symmetry, the plane including the optical axis of the objective lens and perpendicular to the straight line passing through the centers of said pair of reimaging lenses. This focussed point detecting optical system permits changing base length and is capable of favorably detecting focussed points of lenses having various F number. The variable refractive index optical element is a liquid crystal prism so composed as to permit varying voltage to be applied.