The properties of focused fields

The axis of a wave beam has a principal intensity maximum and a number of local additional maxima. It is shown that at points on the axis where the intensity is extremal the curvature of the wavefront is equal to zero. The principal maximum is shifted from the geometrical-optic focus toward the radiating aperture. Approximation formulas are derived for the shift and value of the maximum intensity for an even-symmetric field in the two-dimensional problem and for an axisymmetric field in the three-dimensional problem.