"Just to clear up a little terminology here, what your seeing is really "light falloff" and not vignetting."

Paul,

Do you have a source for that definition? Though what Ara is observing is probably what I know of as 'artificial' or 'optical' vignetting (as I described above) because it disappears when the lens is stopped down, the falloff in illumination according to the cos ^4 (or cos^3 if the Slyusarev/tilting pupil effect is used) is often called 'natural vignetting' as far as I was aware.

Thanks,
Helen