Parfocality between the viewing eyepieces and the camera image is allways very convenient and almost a must with high power (high NA) objectives.
Is it in each of your setups parfocal?
If you place the camera at parfocal position over the phototube without the C mount adapter (or better, without the upper phototube part with a bellows like in your bellows setup) and you still get vignette as other said you need some relay magnification.
Direct projection with a 10X eyepiece (as it seems in your third setup picture) is of course doable, even a classic system, but not the most convenient because you're using the optics well outside its desing.
IMO the best approaches will be:
1. Mount a Nikon CF projective eyepiece (2X or 2.5X) in the phototube at parfocal position and a bellows/tube adapter to hold the camera over it at the right distance (likely the best approach with your microscope)
-something like the Charles Krebs ones:
http://www.krebsmicro.com/microsetup2/index.html
http://micropix.home.comcast.net/~micro ... index.html
2. Mount a Nikon CF visual type high eyepoint 10X eyepiece in the phototube at parfocal position and 50mm camera prime lens, what is called "afocal" setup. You will get 2X relay magnification
- somethig like my first setup:
http://www.photomacrography.net/forum/v ... 9265#99265
3. Mount a good quality teleconverter (2X, but also 1.4X may be convenient) with an empty adapter without any other relay optics. Seems to work very well.
- Discussed here:
http://www.photomacrography.net/forum/v ... 322#146322
http://www.photomacrography.net/forum/v ... 505#146505