GreenSugar, take a look and read the discussion at
http://www.photomacrography.net/forum/v ... hp?t=35350 . The most common place where vignetting occurs, especially with Nikon, is at the back end of the tubes where they enter the camera body. The Nikon bayonet by itself is just barely wide enough for light to reach the corners of a fullframe sensor, when it starts from a point roughly 200 mm in front of that. So, you want the thinnest possible walls = widest clear diameter you can get at at the camera body. The best approach is what's shown in that link: tubes at the body that are built to go directly from a wide tube down to a Nikon bayonet.
Also relevant here is the question that paleblue asked just above yours (but 3 years earlier, and never answered, oops!).
First, answering paleblue's question...
paleblue wrote:If the objective you're using is infinity corrected, then isn't all the light hitting the lens tube parallel? How can the distance between the lens tube and objective matter?
With an infinity objective, all of the light rays from a
single point on the focused subject leave the objective parallel to each other, as if that point were at infinity. But the light rays from one point on the subject are
not parallel to those from a different point on the subject. (If they were, there'd be no way to tell which were which, since every bundle of rays fills the entire rear aperture of the objective.)
The light rays from points away from the center of the field leave the objective at an angle, getting farther from the center with increasing separation. This makes them hit the tube lens at various distances from center. Changing the separation between objective and tube lens changes the distance away from center, for light rays at the same angle.
If the distance away from center is small, then changing it only makes small changes in image quality, by affecting aberrations.
But if the distance away from center becomes too large, all or part of the bundle may get blocked from reaching the sensor, either by failing to enter the tube lens at all, or by entering the tube lens at a distance away from center that causes it to be blocked closer to the sensor. Either of those cases causes vignetting.
So, having the widest possible clear diameter at the camera also gives the most freedom to increase the separation between objective and tube lens. Usually this does not matter, but it could in unusual cases, say if you wanted extra space to insert a beam splitter for through-the-lens illumination.
--Rik