Strong deflection gravitational lensing
  In this work are reviewed several aspects of gravitational lensing produced by astrophysical bodies that strongly curve the spacetime in their vicinity. When an object with a photon sphere (e.g. a black hole) is interposed between a source and an observer, in addition to primary and secondary images due to small deflections of the light rays, there are two infinite sets of images called relativistic, produced by light passing close to the photon sphere, which undergoes a large deflection. The positions and magnifications of relativistic images can be obtained numerically or analytically, in the latter case by an approximate method called strong deflection limit. This method is applied to the study of lensing by non-rotating black holes and also by rotating ones. For the latter, is also of interest to study the deformation of the shadow cast by the black hole, which depends on the rotation parameter. 
Ernesto F. Eiroa
gr-qc