An objective lens for an optical head converges a first laser beam having a
relatively short wavelength and a second laser beam having a relatively
long wavelength on first optical disc (DVD) and a second optical disc
(CD), respectively. A protective layer of the first disc is thinner than a
protective layer of the second optical disc. The objective lens is
compensated for astigmatism with respect to the first beam, which is
incident on the objective lens as an off-axis beam at a predetermined
incident angle, by providing on-axis astigmatism corresponding to wave
front aberration greater than or equal to 0.01.lambda. (rms), and the
objective lens is configured such that the coma is compensated better in a
case where the first laser beam is converged on the first optical disc
than in a case where the second laser beam is converged on the second
optical disc.
