| 摘要 |
An optical system for optical pickup includes a magnification conversion optical element moving along an optical axis direction in accordance with each of optical recording medium recording surfaces, and an objective lens element converging a light beam incident through the magnification conversion optical element, to form a spot on a corresponding one of the recording surfaces, and satisfies the following formula: 4.0×10-4<Mn/(tn-tc)×f<6.0×10-4. Here tc is the thickness [mum] of a recording-medium base material at which a third-order spherical aberration occurring when a parallel light beam is incident on the objective lens element is the minimum, f is the focal length [mm] of the objective lens element, and Mn is the imaging magnification of the objective lens element when a spot is formed through a thickness tn [mum]of the base plate.
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