I hope someone is brilliant and friendly enough to help me.

My immediate interest is the path of light from the rear element of a Nikon 600T f/9 lens when focused at infinity. I need to know the diameter of the disk of light leaving the rear element at 2 inches behind the glass. Also, if this diameter is larger than I hope, then I need to know what affect it has on my 8x10 image---i.e, do I get visible vignetting or just a little insignificant loss of light?

This lens has a telephoto design, with an extended medium-sized rear element. It's P factor (i.e., its pupillary magnification factor, which the ratio of the exit pupil diameter as seen from the rear to the entrance pupil diameter as seen from the front) is approximately 0.72. That means that the apparent size of the rear aperture is 0.72*600/9 = 48mm.

Two possible answers occurred to me: First, maybe I should imagine a 48mm circle inside my Copal 3 shutter and expand it linearly to my image circle on the film plane. Second, maybe I should move that 48mm circle in front of the lens, for this is a telephoto lens, and measure 600mm to the film plane, again expanding it to the size of the coverage on the film plane. The problem is, I am not sure, in fact I even doubt, that there is room for this second shape to fit in the exit glass of the rear element.

Does anybody know?

I have studied mechanical vignetting from compendium shades enough to know that if 10% of the apparent image of the aperture is blocked looking from where the extreme corner of the film will be, then the loss of light in the corner will rarely be noticeable. However, the proper considerations near the rear element might be completely different. Maybe I don't need to worry as much in this region, for maybe a little light is lost but there would no vignetting---I cannot figure it out.

Someone is sure to ask "Why do I want to know this?" Honestly, just curiosity would be enough for me, but in this case, I have a practical reason too: In the thing I am building, I want the metal near the rear element to be cut only once, and I want to leave as much strength in the structure as possible. Furthermore, I would like to learn the general principles, because I have a similar-but-trickier project in mind for my wonderful Telomar 36cm lens.