Patent ID: 7606136

Claim:
An objective lens used for an optical pickup device, the optical pickup device comprising: a light source; and a converging optical system including the objective lens for converging a light beam emitted from the light source to an information recording surface of an optical information recording medium, wherein the optical pickup device is capable of recording and/or reproducing information by converging the light beam emitted from the light source to the information recording surface of the optical information recording medium with the converging optical system, and wherein the objective lens is a plastic single lens and satisfies following formulas: NA≧0.8 (1) 1.0>f>0.2 (2) 0.8< d/f< 1.8 (14) 500≧λ 0 ≧350 (15) 0.40≦( X 1 −X 2)·( N− 1)/( NA·f ·√(1 +|m |))≦0.63 (16) where NA is an image-side numerical aperture of the objective lens, which is required for recording and/or reproducing information to the optical information recording medium, f(mm) is a focal length of the objective lens, λ 0 (nm) is a design wavelength of the objective lens, d (mm) is a lens thickness in an optical axis of the objective lens, X1 (mm) is a distance in an optical axis direction between a plane that is perpendicular to an optical axis and tangent to a top of an optical surface on a light source side and an optical surface on the light source side in a most peripheral portion of an effective diameter (position of the NA on a surface on the light source side to which a marginal light beam is incident), X2 (mm) is a distance in an optical axis direction between a plane that is perpendicular to an optical axis and tangent to a top of an optical surface on an optical information recording medium side and an optical surface on the optical information recording medium side in a most peripheral portion of an effective diameter (position of the NA on a surface on the optical information recording medium side to which a marginal light beam is incident), N is a refractive index of the objective lens at the design wavelength λ 0 , and m is an image formation magnification of the objective lens, wherein X1 is a plus in a case of measuring X1 in a direction of the optical information recording medium with reference to the tangent plane, and minus in a case of measuring X1 in a direction of the light source, and wherein X2 is plus in a case of measuring X2 in a direction of the optical information recording medium with reference to the tangent plane and minus in a case of measuring X2 in a direction of the light source.