Hey roger if you focus you 105mm towards the 1:1 end of the focusing range then you lose the vignetting. You should also focus you 28mm until the inner glass is closes to the rear of the lens, just adjust it a few times and you'll see what I'm talking about.
Roger Sr. wrote:
Yes, I have a 28mm Sigma that I use for reverse mounting it works good but if I use it reversed on the Nikkor 105D lens I get vignetting but great mag.