35
The lens is a Rodenstock Sironar f135mm/5.6, and the back is a Horseman 6x8cm roll film back, originally for a large format camera. I bought a 65mm focusing tubus, flange adapter and copal adapter and printed a ring with the correct focusing markings for this lens. The grip is a standard grip for camera cages. The viewfinder is originally from a different camera, and covers a bit more viewing angle than the lens. The interior of the body is painted with musou black.
Here are some test shots on Tri-X (badly digitized with my phone):
That sounds like a lot more skill than I have :) cool project!
It works quite well for madium to large distances. For anything close you'll need equal parts of skill and luck. ; )