Patent ID: 7079473

Claim:
An objective lens for use in an optical pickup apparatus for conducing recording information for an optical information recording medium and/or reproducing information recorded in the optical information recording medium by converging a light flux from a light source onto an information recording plane of the optical information recording medium, comprising: two positive lenses of a first lens arranged at a light source side and a second lens arranged at an information recording medium side; wherein the objective lens comprises a first surface being a light source side optical surface of the first lens and a third surface being a light source side optical surface of the second lens, and at least two optical surfaces including the first surface and the third surface are made to be an aspherical surface respectively, and the objective lens satisfies the following formulas: NA>0.8 1.2 mm>f>0.3 mm −0.06 >ΔSAG>− 0.24 Δ SAG =( X 1 ′−X 3′)/( NA 4 ·f ·(1 +|m |)) X 1 ′=X 1·( N 1−1) 3 /f 1 X 3 ′=X 3·( N 2−1) 3 /f 2 where NA is an image side numerical aperture of the objective lens necessary for conducting recording and/or reproducing for the optical information recording medium, f is a focal length (mm) of the objective lens, X1 is a positional difference (mm) in an optical axis direction between a plane which contacts a vertex of the first surface and is perpendicular to the optical axis and a most peripheral portion of the first surface within an effective diameter (the most peripheral portion is a position on the first surface at which a marginal ray on the NA enters), wherein the positional difference has a positive sign when the positional difference is measured from the plane toward the optical information medium side and the positional difference has a negative sign when the positional difference is measured from the plane toward the light source side, X3 is a positional difference (mm) in an optical axis direction between a plane which contacts a vertex of the third surface and is perpendicular to the optical axis and a most peripheral portion of the third surface within an effective diameter (the most peripheral portion is a position on the third surface at which a marginal ray on the NA enters), wherein the positional difference has a positive sign when the positional difference is measured from the plane toward the optical information medium side and the positional difference has a negative sign when the positional difference is measured from the plane toward the light source side, m is a magnification of the objective lens, N 1 is a refractive index of the first lens for a designed wavelength, N 2 is a refractive index of the second lens for a designed wavelength, f 1 is a focal length (mm) of the first lens, and f 2 is a focal length (mm) of the second lens.