  In contrast to the case of quasi-monochromatic waves, a focused optical pulse
in the few-cycle limit may exhibit two independent curved wavefronts,
associated with phase and group retardations, respectively. Focusing optical
elements will generally affect these two wavefronts differently, thus leading
to very different behavior of the pulse near focus. As limiting cases, we
consider an ideal diffractive lens introducing only phase retardations and a
perfect non-dispersive refractive lens (or a curved mirror) introducing equal
phase and group retardations. We study the resulting diffraction effects on the
pulse, finding both strong deformations of the pulse shape and shifts in the
spectrum. We then show how important these effects can be in highly nonlinear
optics, by studying their role in attosecond pulse generation. In particular,
the focusing effects are found to affect substantially the generation of
isolated attosecond pulses in gases from few-cycle fundamental optical fields.
