  Recently, Holz & Wald have presented a new method for determining
gravitational lensing effects on, e.g., supernova luminosity versus redshift
measurements in inhomogeneous universes. In this paper, their method is
generalized in several ways: First, the matter content is allowed to consist of
several different types of fluids, possibly with non-vanishing pressure.
Second, besides lensing by simple point masses and singular isothermal spheres,
the more realistic halo dark matter distribution proposed by Navarro, Frenk &
White (NFW), based on N-body simulation results, is treated. We discuss various
aspects of the accuracy of the method, such as luminosity corrections, and
statistics, for multiple images. We find in agreement with other recent work
that a large sample of supernovae at large redshift could be used to extract
gross features of the mass distribution of the lensing dark matter halos, such
as the existence of a large number of point-like objects. The results for the
isothermal sphere and the NFW model are, however, very similar if normalized to
the observed luminosity distribution of galaxies. We give convenient analytical
fitting formulas for our computed lensing probabilites as a function of
magnification, for several redshifts.
