How can I connect the Nikon el-nikkor 50mm f2.8 enlarging lens to my Canon 5D Mark II? What adapter / reversing ring do I need?
The lens in question is designed for a photographic enlarger which is used during film printing to enlarge the negative onto the photo paper. However, there are places that will, in fact, make adapters one of which is Zörk Film and Phototechnic in Germany. They don't really list the product detail, but if you contact them, they can probably let you know if it is possible. I believe the Multi-Focus System uses enlarger lenses.
For interest, Nikon talks about this lens here: http://www.nikon.com/about/feelnikon/recollections/r15_e/index.htm