To emphasize:
Pau wrote:The problem is not the camera lens itself but the lens-objective combo. ... The lens construction, in special the diaphragm plane position also matters.
This is the key point. To avoid vignetting, the rear lens must be designed so that the frame can be filled even when light only comes through the center of the front surface.
Some lenses are designed so that the edges of the frame must get their light through the edges of the front surface. Those lenses will vignette when used with an objective, because the objective provides only a small hole in the center of the lens.
Here are two examples. Both lenses have the same focal length and f-number, but they have very different designs. The magenta block represents the rear of the objective.
The top lens has its diaphragm placed far back, so that light can only reach the edges of the frame when it comes through the edge of the front surface. But the objective gives no light in that area, so the edges of the frame go dark. This combo will vignette.
The bottom lens has its diaphragm placed far forward, so that light reaches the edges of the frame even when it comes just through the center of the front surface. This combo will not vignette.
Note that both lenses work fine by themselves. It is only in combo that you get vignetting.
first confirm that you use the 180mm focussed to infinite and with the diaphragm wide open, and the objective is placed as close as possible to the front lens)
The above are very important. Those are the only two adjustments that will affect the vignetting, once you have chosen the objective and the lens on the camera.
--Rik
