  It is usually assumed that the field isotope shift (FIS) is completely
determined by the change of the averaged squared values of the nuclear charge
radius $\langle r^2\rangle$. Relativistic corrections modify the expression for
FIS, which is actually described by the change of $\langle r^{2
\gamma}\rangle$, where $\gamma=\sqrt{1 - Z^2 \alpha^2}$. In the present paper
we consider corrections to FIS which are due to the nuclear deformation and due
to the predicted reduced charge density in the middle of the superheavy nuclei
produced by a very strong proton repulsion (hole in the nuclear centre).
Specifically, we investigate effects which can not be completely reduced to the
change of $\langle r^2 \rangle$ or $\langle r^{2 \gamma}\rangle$.
