Note that with a symmetric PSF the modulus bars can be omitted. This is strictly true with defocus and spherical aberration, where the wavefront aberration is uniform over the image field of view, and locally for field curvature and astigmatism. With coma and distortion we must take account of the phase function; with distortion the phase function is proportional to f'2x þ f'2y, so that we have an equal shift in the radial direction for all frequencies.