Patent ID: 7929401

Claim:
An objective lens used for an optical information recording/reproducing apparatus for recording information to and/or reproducing information from an optical disc based on a predetermined standard, wherein: the objective lens is made of resin and has a numerical aperture larger than or equal to 0.75; when f (unit: mm) represents a focal length of the objective lens with respect to a wavelength λ (unit: nm) used for recording information to and/or reproducing information from the optical disc, d (unit: mm) represents a thickness of the objective lens along an optical axis of the objective lens, and n represents a refractive index of the objective lens with respect to the wavelength λ, the objective lens satisfies following conditions (1) and (2): 1.00<f<1.50 (1), 1.55 <d·n/f< 1.82 (2); at least a light source side surface of the objective lens is an aspherical surface; when h (unit: mm) represents a height from the optical axis, r (unit: mm) represents a curvature radius of the aspherical surface on the optical axis, κ represents a conical coefficient, and A 4 , A 6 , . . . represent aspherical coefficients larger than or equal to a fourth order, a sag amount SAG which is a distance between a point on the aspherical surface at a height of h from the optical axis and a plane tangential to the aspherical surface at the optical axis is represented by a following equation: SAG = h 2 r 1 + 1 - ( 1 + κ ) ⁢ ( h r ) 2 + A 4 ⁢ h 4 + A 6 ⁢ h 6 + A 8 ⁢ h 8 + … ⁢ ; a gradient (SAG)′ of the aspherical surface is represented by a following equation: ( SAG ) ′ = ⅆ ( SAG ) ⅆ h ; and the objective lens satisfies a following condition (3): 2.80<(SAG1)′ MAX ·n <4.10 (3), where (SAG1)′ MAX represents a maximum gradient of the gradient (SAG1)′ of the aspherical surface at the light source side within an effective beam diameter.