Long time ago I bought a Pentax medium format lens. It's a Pentax SMC 67 165 mm f/2.8. The reason I bought this was to have as low vignetting as possible. Later I bought the Sigma APO 150 mm f/2,8. Very good but a bit high vignetting.
After this the Pentax have collected a lot of dust. But why not give it one more chance?
Here you can see some graphs over the vignetting where I compare the Sigma with the Pentax:
http://www.astrofriend.eu/astronomy/...vs-pentax.html
Later I shall also do some test how sharp it is, now with a live view camera it's much easier to get it in perfect focus, I think I failed with this earlier.
Any other out there using medium format lenses?
/Lars