We develop the spacetime approach to gravitational lensing by spherically symmetric perturbations of flat, cosmological constant-dominated Friedman-Robertson-Walker metrics. The geodesics of the spacetime are expressed as integral expressions which are used to examine the formation of multiple images and the observed shapes of non-point sources. We develop the lens mapping from the spacetime perspective, and use the Jacobian of the mapping to explain the observed image shapes. Approaching the geodesic equations as ordinary differential equations, we demonstrate the development of wave front singularities and time delays between light ray signals. This work demonstrates that the widely used thin lens approximation can be replaced with more robust techniques aligned with general relativity.
Kling et al. (Mon,) studied this question.