I bought this lens hoping it is a sleeper. Well, it isn't. It is heavy ,though. It is has an m42 mount so I tested it on my T2i. I wouldn't consider using it on my M3. Handling on the T2i without AF confirmation is difficult. It is simply too soft af f2.8. It is soft at f2.8. It is usable from f4 but it never gets sharp. It was an off-brand lens sold by a dept. store. It is multicoated, but that only helps with chromatic abberations. Fringing is well controlled at f2.8, but it's so soft at F2.8 that all colors and defintion are muted. At f4 I could see blue fringe in bright areas butit is minor. Contrast is low, yet there is no shadow detail either. If you are shooting film, I suggest shooting b/w. If you are using a sub-frame D-slr, I suggest shooting RAW and post processing. Colors are a bit flat. Make sure your white balance is set properly.
Distortion is noticable on the T2i. It is a complex distortion, too. I must assume it is obvious on full frame bodies. Avoid architectural scenes. There is also visble vignetting, even on my T2i. It will be obvious on a full frame body. At f2.8 it is noticeable. At f4 is starts to clear up. F4 is the largest aperture I would use with this lens. At any aperture you will get flare. Any light crossing the lens from the side will produce significant flare. Use a lens hood at all times. Bokeh is pleasing and bokeh fringing is well controlled.