  Light rays passing very close to a black hole may experience very strong
deviations. Two geometries were separately considered in the recent literature:
a source behind the black hole (standard gravitational lensing); a source in
front of the black hole (retro-lensing). In this paper we start from the Strong
Field Limit approach to recover both situations under the same formalism,
describing not only the two geometries just mentioned but also any intermediate
possible configurations of the system source-lens-observer, without any
small-angle limitations. This is done for any spherically symmetric black holes
and for the equatorial plane of Kerr black holes. In the light of this
formalism we revisit the previous literature on retro-lensing, sensibly
improving the observational estimates. In particular, for the case of the star
S2, we give sharp predictions for the magnitude of the relativistic images and
the time of their highest brightness, which should occur at the beginning of
year 2018. The observation of such images would open fascinating perspectives
on the measure of the physical parameters of the central black hole, including
mass, spin and distance.
