Patent ID: 8531926

Claim:
An optical system for an optical pickup that converges an incident light beam onto an optical recording medium, the optical recording medium having at least three recording surfaces located parallel to each other, the optical system converging the incident light beam through an incident surface of a base material of the optical recording medium onto any of the at least three recording surfaces to form spots corresponding to the recording surfaces, the optical system comprising: a magnification conversion optical element moving along an optical axis direction in accordance with each recording surface; and an objective lens element converging a light beam incident through the magnification conversion optical element, to form a spot on a corresponding recording surface among the recording surfaces, wherein the optical system satisfies the following formula, 4.0×10 −4 <Mn/ ( tn−tc )× f< 6.0×10 −4 (1), where tc is a thickness [μm] through the recording-medium base material at which, with the objective lens element forming a parallel light beam incident thereon into a spot, third-order spherical aberration is minimum, tc therein being a thickness from the base-material incident surface to a plane different from any of the at least three recording surfaces, f is the focal length [mm] of the objective lens element, and Mn is the imaging magnification of the objective lens element when a spot is formed through a thickness tn [μm] of the recording-medium base material, tn therein being, distinct from the thickness tc, a thickness of the base material to any of the at least three recording surfaces.