>>3543155M39 adapters are slim but M39 lenses don't have anywhere near the same close focusing distance as M42 mostly due to the restrictions of rangefinder cameras. So you lose the ability to get close which isn't a problem with M42 lenses since they're adapted from SLRs. That's my experience anyway, using an Industar 61 adapted to my Nex 5n it was kind of a bummer to not be able to get close with it.
>>3543186Most vintage lenses, certainly the cheap shit ones that /p/ buys, are soft around the edges. So when adapted you only get the sharp parts of the lens. There are exceptions to that rule but some lenses genuinely are sharper adapted to crop due to losing the soft mushy corners.
