Patent ID: 8116188

Claim:
A plastic objective lens configured to collect a light beam with at least a wavelength λ on a recording layer of an optical disk and used in an optical pickup for recording and/or reproducing an information signal by illuminating a light beam on the optical disk having one or more of recording layers in an incident direction of the light beam, wherein: an amount of 3rd order comatic aberration [λrms], which is caused when the objective lens is tilted by 1 degree, is a lens tilt sensitivity; the lens tilt sensitivity at an image height sensitivity of 0 is positive; and wherein under an environment in which a temperature range is 0° C. to 70° C. and a range of the wavelength λ of the light beam is 400 to 410 nm, a maximum lens tilt sensitivity at an n-th layer is ΔW LT — Max — Ln ; a minimum lens tilt sensitivity is ΔW LT — Min — Ln ; f is a focal length [mm] of the objective lens at the wavelength λ; Δt L0-Ln is a through-thickness distance [mm], at which a thickness of a cover layer is the maximum, from a 0th layer to an n-th layer of the one or more recording layers; and a numerical aperture NA at the wavelength λ satisfies the relationship of NA>0.8, and the relationships of the following Expressions (1) and (2) are satisfied in all the recording layers of the one or more recording layers: 0.034· f≦ΔW LT — Max — Ln ≦0.25+2.0 ·Δt L0-Ln (1), and −0.053 ·f≦ΔW LT 13 Min — Ln ≦0.25+2.0 ·Δt L0-Ln −0.068 ·f (2).