Steve, why not get a pencil, and draw your focal length on a long long piece of paper and trace out the ray path and then use the measurements of the backfocus and filter placemnt in front of the camera to see exactly how much vignetting/cuttoff your going to create.
This is a true life measurment, and will give you not only a visual representation of the result, but also answer your questions on any other f ratio.
Theo
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