Patent ID: 8125876

Claim:
An objective lens, wherein: the objective lens is made of plastic; the objective lens has a numerical aperture equal to or greater than 0.80; and an expression  SA ⁢ ⁢ 5 ⁢ ( L ) _ ⁢ Δ ⁢ ⁢ L ⁡ ( 1 - p M p L ) + SA ⁢ ⁢ 5 ⁢ ( T ) _ ⁢ Δ ⁢ ⁢ T ⁡ ( 1 - p M p T )  < 0.01 ⁢ ⁢ λ rms is satisfied, wherein: p L is a ratio of a layer-based fifth-order spherical aberration to a layer-based third-order spherical aberration, the layer-based fifth-order spherical aberration and the layer-based third-order spherical aberration being caused by a change in a thickness of a cover layer of an optical recording medium; p T is a ratio of a temperature-based fifth-order spherical aberration to a temperature-based third-order spherical aberration, the temperature-based fifth-order spherical aberration and the temperature-based third-order spherical aberration being caused by a change in temperature of an environment of the objective lens; p M is a ratio of a magnification-based fifth-order spherical aberration to a magnification-based third-order spherical aberration, the magnification-based fifth-order spherical aberration and the magnification-based third-order spherical aberration being caused by a change in incident magnification; expressions 0.83<( p M /p L )<1.17 and 0.83<( p M /p T )<1.17 or expressions 0.80<( p M /p L )<1.20 and 0.86<( p M /p T )<1.14 are satisfied; SA 5 (L) is a slope of the layer-based fifth-order spherical aberration; SA 5 (T) is a slope of the temperature-based fifth-order spherical aberration; ΔL is the change in the thickness of the cover layer; and ΔT is the change in the temperature of the environment of the objective lens.