Hi, I have an uncoated ( ca 1939 ) Biotar on my Robot 2, scale focusing only, at f/2.0 pics aren't really sharp, also lack contrast. My postwar coated Xenon 40mm is much better. The Biotar is a collectible and therefore demands high prices. I tried it with an adapter on M39 Barnacks, but it vignettes on 24x36 and pics aren't remarquable. On the Sony Nex (APS-C format sensor ) results are quite good. While most C-mount lenses will vignette, or give a circular image on the Nex, the M26 Robot lenses are usable. The later Robot Royal lenses would be an even better choice, if you can find an adapter.