Quote Originally Posted by titrisol
The soviets copied the 39mm screw mount, but at the beginning it had adifferent thread number than the german mount. If your NIR lens is old enough you may have a tough problem there.
The Mir has been presented to the Public in about 1958 and it has the correct size and thread pitch to qualify for M39... just the wrong lens/film flange distance. Basically that Zenit is a Zorkii with a mirror added to the front... then they realized that 39mm arenŽt wide enough for many non-Retrofocus lens designs and widened it to Start bajonet mount and M42 (which was already in use by Praktica - but on Soviet pressure. Rumours are that they had an order on SLRs from the Soviets and worked hard to fulfill the schedule... there wasnŽt enough time to implement a bayonet mount, so the M42 design was chosen).

To use it with the Bessa or any other RF youŽd need a step up ring of 45,46mm minus 28,8mm... and machine it in tolerances of +/- 0.02mm.

Certainly not impossible, but a major hassle. YouŽll loose RF coupling, youŽll get an ealy non-retrofocus design... a J12 in M39 is better.. ;-)