| 摘要 |
An optical system includes a laser light source; an objective lens for converging luminous flux emitted from the laser light source onto an information recording surface of an optical information recording medium through a transparent substrate of the optical information recording medium; and lateral magnification change device for changing a lateral magnification of the objective lens from a first lateral magnification m1 to a second lateral magnification m2 which is less than m1 according to a thickness of the transparent substrate. An amount of an offense against sine condition SC(m1:NA2) represented by the following expression for NA2 at the first lateral magnification m1 of the objective lens satisfies the following conditional expression,0.06>/=SC(m1:NA2)/f>/=0.002andSC(m1:NA2)=d2xcos (u2)/NA2-(1-m1)xfwherein f represents a focal length of the objective lens, NA2 represents a numerical aperture of the objective lens at the second lateral magnification m2, d2 represents a height of a ray on a principal plane of the objective lens from an optical axis for the numerical aperture NA2 at the first lateral magnification m1, and u2 represents an incident angle of the ray to the objective lens for the numerical aperture NA2 at the first lateral magnification m1.
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