Patent ID: 8018816

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, the objective lens being made of resin and having a numerical aperture larger than or equal to 0.75; the objective lens satisfying a condition: 0≦|SC| MAX /f< 0.002 (1) where f denotes a focal length (unit: mm) 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, and |SC| MAX denotes a maximum value of an absolute value an offence against a sine condition defined in a range larger than or equal to 0% and smaller than 90% of an effective beam radius with respect to an optical axis of the objective lens, the objective lens being configured such that the offence against the sine condition at a 100% point of the effective beam radius is smaller than the offence against the sine condition at a 90% point of the effective beam radius, at least a light source side surface of the objective lens being formed to be an aspherical surface, wherein: when SAG denotes a sag amount defined as a distance between a point on the aspherical surface at a height of h (unit: mm) from the optical axis and a plane tangential to the aspherical surface at the optical axis, r denotes a curvature radius (unit: mm) of the aspherical surface on the optical axis, κ denotes a conical coefficient, and A 4 , A 6 , . . . denote aspherical coefficients larger than or equal to the fourth order, a gradient (SAG)′ of the aspherical surface at the height h is expressed by: ( SAG ) ′ = ⅆ ( SAG ) ⅆ h ; where, SAG = h 2 r 1 + 1 - ( 1 + κ ) ⁢ ( h r ) 2 + A 4 ⁢ h 4 + A 6 ⁢ h 6 + A 8 ⁢ h 8 + … ⁢ ; ⁢ and the objective lens is configured to satisfy a condition: 1.40<( n− 1)·(SAG1)′ MAX ·/NA 2 <1.95 (2), where (SAG1)′ MAX represents a maximum gradient of the light source side surface within an effective beam diameter, n denotes a refractive index of the objective lens at the wavelength λ, and NA denotes the numerical aperture of the objective lens defined on a side of the optical disc when the wavelength λ is used.