Comparative studies of lensing methods

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. However, observationally significant errors were found when the thin lens method was applied to lensing configurations with two lenses. In these cases, the iterative method remained accurate beyond observational limits.