I asked this question on LinkedIn a while back but didn't get many relevant answers. In fact, most responders thought I was asking about Seidel aberrations (AKA 3rd order aberrations). Let me be clear here that I'm asking about Seidel coefficients and not Seidel aberrations in general. Just for the background sake when the wavefront polynomial is derived for a symmetrical optical system, the polynomial coefficients are called wavefront coefficients. Physically, these coefficients are optical path differences which is very useful information. Suppose we have just spherical aberration (non-zero coefficient for r^4) and we want to find its corresponding Seidel coefficient. This can be done by just multiplying it by 8. Other terms have other conversion factors which is fine, but what I'm hung up on is whether Seidel coefficients are simply there for historical purpose or do they actually have any physical meaning. Can anybody tell me whether or not there's any underlying rationale for Seidel coefficients, or are we free to redefined coefficients for describing 3rd order aberrations purely for our own enjoyment?