I was given an old m42 screw mount lens very recently. It's a 'Paragon 135mm f/2.8', which sounds like a brand name made up by the importer. It looks a bit silly on my GF1, and due to the lack of viewfinder or IBIS, it's actually really difficult to frame and focus shots in practice. It's not that great wide open unfortunately, but nice to have for emergencies. Stopped down things improve, I imagine I'd probably want to shoot it at f/5.6 most of the time. Chromatic aberrations are pretty bad wide open, and only improve slightly when stopping down. Samples below. Most interesting thing around to test it with was the cat. Spoiler Shot at f/5.6 it's actually quite sharp: Same shot at 100% At f/2.8 the contrast is really low and the sharpness suffers quite a bit.