A wavelet from the surface element dS arrives in a point (x, y) on the image plane with a phase kl, where l is the optical path length (OPL) from (xp, yp) to (x, y). When the pupil wavefront is spherical with radius r, the OPL from each point on the wavefront to the axial point (0,0) of the image plane is equal to n' r. In the following we take n' = 1 for convenience. The OPL to a point (x, y) in the neighborhood of (0, 0) is, to a first order approximation, equal to