  We discuss the problem of identification of coupling constants, which
describe interactions between photons and space-time curvature, using exact
regular solutions to the extended equations of the nonminimal Einstein-Maxwell
theory. We argue the idea that three nonminimal coupling constants in this
theory can be reduced to the single guiding parameter, which plays the role of
nonminimal radius. We base our consideration on two examples of exact solutions
obtained earlier in our works: the first of them describes a nonminimal
spherically symmetric object (star or black hole) with regular radial electric
field; the second example represents a nonminimal Dirac-type object (monopole
or black hole) with regular metric. We demonstrate that one of the inflexion
points of the regular metric function identifies a specific nonminimal radius,
thus marking the domain of dominance of nonminimal interactions.
