A systematic theoretical and experimental study of focusing femtosecon
d light pulses by single lenses is presented. By evaluation of the dif
fraction integral the interplay of spherical and chromatic aberration
is shown to determine the temporal as well as the spatial intensity di
stribution in the focal region of a lens. Conditions are derived under
which the effect of spherical aberration dominates. Here a temporally
unbroadened, in-focus pulse occurs while the spatial distribution is
that expected from an annular lens aperture. If chromatic aberration i
s the major aberration, the in-focus pulses are considerably broadened
. Both effects could clearly be measured with 100-fs pulses using a mo
dified Michelson interferometer. In an intermediate parameter range bo
th chromatic and spherical aberration contribute to the pulse broadeni
ng and to the spatial intensity pattern in a given plane in the focal
region. From Fresnel diffraction we expect a weak-intensity distributi
on to precede the in-focus pulse on the axis. Another weak intensity p
eak is produced by the pulse traveling on axis. By measuring its separ
ation from the main pulse in the marginal focal plane one can estimate
the aberration parameters.