  We reformulate the transport equation which determines the size, shape and
orientation of infinitesimal light beams in arbitrary spacetimes. The behaviour
of such light beams near vertices and conjugate points is investigated, with
special attention to the singular behaviour of the optical scalars. We then
specialize the general transport equation to the case of an approximate metric
of an inhomogeneous universe, which is a Friedmann metric `on average' with
superposed isolated weak matter inhomogeneities. In a series of well-defined
approximations, the equations of gravitational lens theory are derived.
Finally, we derive a relative optical focusing equation which describes the
focusing of light beams relative to the case that the beam is unaffected by
matter inhomogeneities in the universe, from which it follows immediately that
no beam can be focused less than one which is unaffected by matter clumps,
before it propagates through its first conjugate point.
