We study spherical pulse like families of solutions to semilinear
wave equations in space time of dimension 1+3 as the pulses focus
at a point and emerge outgoing. We emphasize the scales for which
the incoming and outgoing waves behave linearly but the
nonlinearity has a strong effect at the focus. The focus crossing
is described by a scattering operator for the semilinear equation,
which broadens the pulses. The relative errors in our approximate
solutions are small in the $L^\infty$ norm.