  Most of optical gravitational lenses recently discovered in the Sloan Digital
Sky Survey Quasar Lens Search (SQLS) have two-images rather than four-images,
in marked contrast to radio lenses for which the fraction of four-image lenses
(quad fraction) is quite high. We revisit the quad fraction among optical
lenses by taking the selection function of the SQLS into account. We find that
the current observed quad fraction in the SQLS is indeed lower than, but
consistent with, the prediction of our theoretical model. The low quad fraction
among optical lenses, together with the high quad fraction among radio lenses,
implies that the quasar optical luminosity function has a relatively shallow
faint end slope.
