I thought I'd lay hands on a mess around with lens, but sadly didn't think it through so well.  I assume I need to step up from M39 to M44 then to a M44 to Nikon converter?  
My primary question is that I've read in some places that the lens can bump the mirror.  Is there fact to this and or is there a work around?  I did find the wonderful thread on painting with lenses, but could not quite locate this information.  I assume while it's going to be clipped, this will also work on a D7100?
Any help is greatly appreciated.  Thanks.