Does This Look Like a Pincushion to You?

I remember studying aberrations in my first undergraduate optics course. I learned that distortions are third-order aberrations independent of the pupil coordinates through which light rays pass and depend only on image height. If the coefficient for the distortion is negative, a rectangular grid imaged by a lens with this type of aberration appears magnified in the center and bowed-out at the sides—a barrel distortion. If the coefficient for the distortion is positive, the grid appears magnified at the edges with sharp, pointed corners—a pincushion distortion.