I bought a 135mm El-Nikkor enlarging lens to replace the Omegeron lens that came with my D5. I was figuring I could just unscrew the old lens, and screw in the new lens. But the retaining ring that held the Omegeron will not screw all the way down on the Nikkor, so that it is not held tightly on the lens board. There's a good 2mm of space, maybe 4-5 turns of threading. It's a flat lensboard and and a chrome-bodied Nikkor, of that makes a difference. Do I need a different size ring? The two lenses look like they take the same size ring.