| 摘要 |
When an extra-high density optical disc, a DVD having a recording density lower than that of the extra-high density optical disc, a CD having a recording density lower than that of the DVD, and a combined optical recording medium in which the extra-high density optical disc, DVD, and CD are appropriately combined and integrally stacked are selectively recorded or reproduced by an objective lens whose numerical aperture (NA) is set to 0.75 or more, the objective lens satisfies the following expression: t<a.f+b wherein t: an axial thickness of the objective lens, f: a focal distance of the objective lens, a: a coefficient, and b: a constant, and the coefficient a and the constant b satisfies that a=2.02, b=-1.94; a=2.08, b=-1.84; or a=2.11, b=-1.77.
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