  When the the shadow image of a supermassive black hole in the center of M87
was obtained a new era in the observational astrophysics began. The resolution
improvement would allow to test extended gravity models at event horizon
scales. We develop the method allowing to take into account the black hole
rotation in black hole shadow modelling. It is demonstrated that the shadow of
a black hole with a diameter smaller than $4R_{Sw}$ ($R_{Sw} = 2M$) cannot be
represented in the terms of pure Kerr-Newman metric. Therefore, the detection
of a shadow of such size would indicate that further expansions must be taken
into account. This fact could give new indications on the detailed form of the
theory of gravity at the considered scales.}
