  The lensing at large deflection angles caused by a Schwarzschild black hole
for the case of a nonminimal coupling between gravitation and electromagnetism
is examined. We show that photons follow an effective geometry, which displays
an effective photon sphere. For the case in which the source, lens and observer
are aligned, so that relativistic Einstein rings are formed, the dependence of
the angular separation $\delta\theta$ between the first and second ring with
the relevant coupling parameter is calculated. We argue that such a separation,
which may be measured by telescopes that will be operative in the near future,
may set an upper and a lower limit for the coupling parameter.
