Sorry about this one for those that are not interested ... Clarification for those that asked: > The occurrence of pincushion or barrel distortion is closely > related to field curvature. Field curvature produces a curved, but otherwise corrected image surface (within the limits set by Petzval's theorem). The selection of a compromise image plane produces a **geometric** distortion (and an increase in the point spread function), that mixes with the true "distortion".(See Conrady's book on optical design for more info). Andy Shaw
