Figure 1: (a) Values of the parameter versus the spin , for which the central object is a black hole. The shaded region marks the excluded part of the parameter space where this object is a naked singularity. The dashed line corresponds to a Kerr black hole [34]. (b) Trajectories of photons lensed by a black hole with a (counterclockwise) spin  M for several values of the parameter . The shaded region corresponds to the event horizon of a Kerr black hole of equal spin.