  When the gravitational lensing potential can be approximated by that of
circularly symmetric system affected by weak perturbations, it is found that
the shape of the resulting (tangential) caustics is entirely specified by the
local azimuthal behaviour of the affecting perturbations. This provides a
common mathematical groundwork for understanding problems such as the
close-wide (d <-> 1/d) separation degeneracy of binary lens microlensing
lightcurves and the shear-ellipticity degeneracy of quadruple image lens
modelling.
