Abstract

In an optical head, laser beams are converged by an objective lens and projected on a light-receiving surface of an optical disk. The laser beam is reflected by the light-receiving surface, converged by the objective lens into parallel laser beams, and directed to a light shielding plate. Only the laser beam passing through the region spaced from the optical axis is allowed to pass by the light shielding plate, and part of this laser beam is converged by a convergent lens and projected on photo sensitive regions of the photo detecting unit. The objective lens has a convergent point and an inherent maximum defocusing allowance with reference to this convergent point. The photo sensitive regions have a total area larger than that of a circle having a radius Rd.wherein F is a distance between a principal point of the objective lens which is at the side of the light-reflecting surface and the convergent point of the objective lens, r is a radius of an exit pupil of the objective lens, m is a multiplication of the beam spot formed on the photo sensitive regions, and f is a focal length of the optical system.