We report on an analytical formulation for evaluating the amplitude di
stribution along any line directed toward the geometrical focus of a s
pherical wave front that passes through a rotationally nonsymmetric di
ffracting screen. our formula consists of two factors. The first facto
r involves the one-dimensional Fourier transform of the projection of
the screen function onto the off-axis line. The second factor depends
on the inverse distance to the screen and permits us to recognize the
existence of focal shift along off-axis lines.