Image distortion from optical scalars in nonperturbative gravitational lensing

In a previous article concerning image distortion in nonperturbative gravitational lensing theory we described how to introduce shape and distortion parameters for small sources. We also showed how they could be expressed in terms of the scalar products of the geodesic deviation vectors of the source’s pencil of rays in the past light cone of an observer. In the present work we give an alternative approach to the description of the shape and distortion parameters and their evolution along the null geodesic from the source to the observer, but now in terms of the optical scalars (the convergence and shear of null vector field of the observer’s light cone) and the associated optical equations, which relate the optical scalars to the curvature of the spacetime.