Predictions of the standard thin lens approximation and a new iterative
approach to gravitational lensing are compared with an ``exact'' approach in
simple test cases involving one or two lenses. We show that the thin lens and
iterative approaches are remarkably accurate in predicting time delays, source
positions and image magnifications for a single monopole lens and combinations
of two monopole lenses. In the cases studied, the iterative method provided
greater accuracy than the thin lens method. We also study the accuracy of a ``2
lens, single lens plane model,'' where two monopole lenses colinear with the
observer are modeled by a mass distribution in a single lens plane lying
between them. We see that this model can lead to large inaccuracies in
physically meaningful situations.
A previous version of this paper was published as Phys.Rev.D62, 024025,
(2000) with errors in the computation of two lens comparisons. This paper
corrects these errors and presents new conclusions which differ from the
previous version.Comment: 23 pages, 15 figures, corrected versio
