Patent ID: 7433291

Claim:
An objective lens used for recording data to and/or reproducing data from a plurality of types of optical discs having different thicknesses of cover layers, the plurality of types of optical discs including a first optical disc and a second optical disc having a cover layer thicker than that of the first optical disc, a first light beam being used for recording or reproducing operation for the first optical disc, a second light beam being used for recording or reproducing operation for the second optical disc, a wavelength of the second light beam being longer than that of the first light beam, the objective lens satisfying a condition: 0.02< M 2− M 1<0.15 (1) where M 1 represents a magnification of the objective lens when the recording or reproducing operation for the first optical disc is performed using the first light beam, and M 2 represents a magnification of the objective lens when the recording or reproducing operation for the second optical disc is performed using the second light beam, at least one of lens surfaces of the objective lens comprising a diffracting structure having a plurality of annular zones configured to correct spherical aberrations for both of the first and second optical discs.