Image distortion in nonperturbative gravitational lensing - art. no. 023006

We introduce the idea of shape parameters to describe the shape of the penc il of rays connecting an observer with a source lying on his past light con e. On the basis of these shape parameters, we discuss a setting of image di stortion in a generic (exact) spacetime, in the form of three distortion pa rameters. The fundamental tool in our discussion is the use of geodesic dev iation fields along a null geodesic to study how source shapes are propagat ed and distorted on the path to an observer. We illustrate this nonperturba tive treatment of image distortion in the case of lensing by a Schwarzschil d black hole. We conclude by showing that there is a nonperturbative genera lization of the use of Fermat's principle in lensing in the thin-lens appro ximation.