I think that this is not entirely true. 75 and 50mm stand-alone f/1.8
What users write in the review, you must match the position in the lens adjustment at infinity (or close) and a sharp image on the tube. Otherwise, the picture becomes softer around the edge. Although it may have something to do with my next comment.
Likewise. The closer the lens (glass) is to the tube (more precisely to the out hole), the less vignetting. In my case, there are additional option. I can cut the right side of the threaded bronze c-mount adapter to a minimum. Photo. In order to further reduce vignetting. Whether this makes sense, we'll see later.
This is not the case for 75 mm. I don't see vignetting using 2" filters.
In any case, I am moving in the direction of decreasing the focal length of the lens and my next step will be CF25 mm (package on the way).
Should not be concerned about the shine of the internal thread of the adapter. In any case, Televue doesn't care.
My comments are true and correct as written.