Yashinon DS 28mm f2.8
Yashica made 3 lines of lenses for m42 mount. DX, DS and DS-M. DX lenses were single coated and not tack sharp but had a nice smooth bokeh. DS lenses were multicoated, sharp and offered a soft bokeh. DS-M lenses were tack sharp but the bokeh was a little harsh. I prefer the DS line. As you can see from the image below, they are exceptional. The 28mm 2.8 is sharper than a Yashica 28mm f2.8 ML and has better bokeh. If you don't need AF, pick one up, you won't be disappointed. Chromatic abberations are not noticeable and it is sharp from f2.8 to f16.