if i can use bending of a caustic point as an example....

The motion of a caustic point for an axial object point is studied while the lens is perturbed by a generalized bending. It is shown that, to third order, the direction of motion is constant. The magnitude of the motion is proportional to the change in the third-order aberration coefficient. An application to the optical design process is indicated.

there you have it. Clear as mud. Hope this helps

Posted By: Pixelman, Mar 4, 12:10:53