Applicability of the parabolic approach (PA) to solution of field problems near the geometric focus is considered. A version of quantitative criterion is given. It was shown that the discrepancy between the exact solution and the PA:
a) generally speaking is always present that is connected with the pemature truncation of series for the phase of the Green's function and boundary field;
b) is unessential if the boundary field and the Green's function has a spherical wavefront;
c) can reach considerable value if that and/or other wavefront becomes nonspherical; and,
d) is the larger the more characteristic boundary and focal size differ due to beam focusing.