Pincushion and barrel distortion of lenses using arc lamp.

Sometimes a lens will be ground inexactly so that focused images will be distorted: the image shape does not exactly correspond to the object shape. If a lens has too steep a curvature in the middle (or if the lens is very thick), the result is barrel distortion. If the lens is too flat in the center, the result is pincushion distortion. In this demonstration, lenses are arranged to show what barrel and pincushion distortions look like. By moving a screen object from point 1 to point 2, the image distortion changes from pincushion to barrel. See figure 1. Pincushion and barrel distortion of lenses using arc lamp.