[Yashinoff]

There was a very small 50/2.8 with front element focusing which I think was a Rikenon. For an M42 camera I would recommend a 55/1.8 or 55/2 S-M-C-T or SMC. These are very well coated, not too large and very sharp.

I have one, and it's a very cheap lens with not so good results. Yashinon and Mamiya also made 2.8/50 lenses which are pretty compact. I have a Petri 2.8/45 which is very small indeed. Hardly anything is smaller than the manual diaphragm CZJ tessars, but using a manual diaphragm lens is such a huge PITA that I'd consider one of the slightly larger pre-set or semi-automatic lenses.