Objective lens for recording and reproducing for use in an optical information recording medium

DETAILED DESCRIPTION An object of the invention is to make it possible that recording and reproducing by means of optical disks having different substrate thicknesses are conducted in a single optical pickup device, and to maintain the performance of the optical pickup device by correcting aberration with lateral magnification of each objective lens taking off-axis characteristics (image height characteristics) and tilt characteristics into consideration when changing magnification and thereby keeping balance of off-axis characteristics even when the lateral magnification of the objective lens is changed.
An optical system is composed of; a light source; an objective lens which converges a light flux emitted from the light source on an information recording surface of an optical information recording medium through a transparent substrate of the optical information recording medium; and a means for changing the lateral magnification of the objective lens itself from the first magnification m.
sub.
1 to the second magnification m.
sub.
2 (m.
sub.
1 >m.
sub.
2), depending on the thickness of the transparent substrate of the optical information recording medium, and, offense against sine condition SC (m.
sub.
1 :NA.
sub.
2) represented by the following expression for NA.
sub.
2 at the first lateral magnification m.
sub.
1 for the objective lens satisfies the following conditions.
0.
06.
multidot.
f.
gtoreq.
SC(m.
sub.
1 :NA.
sub.
2).
gtoreq.
0.
002.
multidot.
f SC(m.
sub.
1 :NA.
sub.
2)=d.
sub.
2 .
multidot.
cos (u.
sub.
2)/NA.
sub.
2 .
multidot.
(1-m.
sub.
1).
multidot.
f f: Focal length of the objective lens NA.
sub.
2 : Numerical aperture of the objective lens at the second lateral magnification m.
sub.
1 d.
sub.
2 : Height of ray of light on a principal plane of the objective lens from the optical axis on the object side for NA.
sub.
2 at the first lateral magnification m.
sub.
1 u.
sub.
2 : Incident angle against the objective lens for NA.
sub.
2 at the first lateral magnification m.
sub.
1 The objective lens is available in the type of glass combination lens (FIG