Patent ID: 7164645

Claim:
An optical system for use in an optical pickup apparatus, comprising: an expander lens including at least two lens groups and to convert a diameter of a light flux; and an objective lens to converge the light flux from the expander lens; wherein the following formula is satisfied: W1 CM >W2 CM where W1 CM is a coma aberration (λrms) of a converged light spot when an off-axial light flux of a wavelength λ (nm) emitted so as to converge at a position distant in a vertical direction from the optical axis of the objective lens by an arbitrary distance Y 1 (mm) not larger than 0.05 (mm) comes into the objective lens, and W2 CM is a coma aberration (λrms) of a converged spot when an off-axial light flux of the wavelength λ (nm) emitted so as to converge at a position distant in a vertical direction from the optical axis of the objective lens by the arbitrary distance Y 1 (mm) comes through the expander lens into the objective lens on a condition that the optical axis of all lens groups constructing the expander lens is arranged so as to conform with the optical axis of the objective lens.