Unfortunately Peter, the things are much more complicated in the afocal photography than you see it. I mistakingly took you for someone who knew it...
In the afocal photography, the vignetting depends on many factors, such as the eyepiece eye relief, the position of the diaphragm of the taking lens in relation to the exit pupil of the eyepiece, the film format, the focal length of the camera lens etc.
If you want to study it more I suggest you see the easy but good book of "Astrophotography for amateurs" by Michael A. Covington. Cheers!
Bookmarks