Patent ID: 7417935

Claim:
An optical system for use in an optical pickup device for recording and/or reproducing information on an optical recording media, the optical system having an optical axis and comprising: an objective system for converging a light beam emitted from a light source on the optical recording media; an optical aberration correcting element comprising two lenses arranged on an optical path between the light source and the objective system and having an adjustable distance in the direction of the optical axis; wherein the optical aberration correcting element comprises a ring-shaped phase structure formed in at least one lens surface of the optical aberration correcting element, and wherein the ring shaped phase structure is non-periodic and comprises a plurality of concentric ring surfaces k (k=1. . . N), and each of the ring surfaces has a step in the direction of the optical axis between the adjacent ring surface; and wherein the ring-shaped phase structure is provided on a surface in the optical aberration correcting element nearest to the objective system and satisfies the following expression: D/rL< 0.1 wherein D represents a diameter of the entrance pupil of the objective system; and rL represents an absolute value of curvature radius of the surface nearest to the objective system.