  Distances play important roles in cosmological observations, especially in
gravitational lens systems, but there is a problem in determining distances
because they are defined in terms of light propagation, which is influenced
gravitationally by the inhomogeneities in the universe. In this paper we first
give the basic optical relations and the definitions of different distances in
inhomogeneous universes. Next we show how the observational relations depend
quantitatively on the distances. Finally, we give results for the frequency
distribution of different distances and the shear effect on distances obtained
using various methods of numerical simulation.
