  Event Horizon Telescope (EHT) collaboration observed ring images called the
shadows of M87* and Sagittarius A* (Sgr A*), which are supermassive objects in
M87 and our galaxy, respectively, and their general relativistic
magnetohydrodynamic simulations of black holes imply that the observed rings
are formed by the gravitational lensing of synchrotron radiations from a hot
plasma near outside of supermassive black holes. EHT collaboration gave
constrains on the electrical or alternative charges of M87* and Sgr A* under an
assumption that the radius of the observed ring should be proportional to the
changing rates of photon spheres by the charges. Since the validness of this
assumption is not sure, it is worth to checking the same constraints under
another assumption. On this paper, we consider the changing rates of not only
the photon spheres but also lensing rings in a simple model and we test whether
aforementioned constrain is robust. We conclude that EHT collaboration's
constraints based on the changing rates of the photon spheres can be relaxed
compared to that based on the changing rate of the lensing rings while we do
not claim that the observed rings are formed by the photon spheres and the
lensing rings in our simple model. We concentrate on Reissner-Nordstr\"{o}m
black hole spacetimes in this paper, but our result implies the relaxation of
the bound of the charge parameters on other black hole spacetimes.
