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Hi all, I need your expert experience again. I just bought a Nikon 150mm enlarging lens and I don't know what size lens board opening it takes to mount it. I had thou ght it was a 39mm but it is much bigger. In fact I have a 39mm lens board for a Beseler 45M enlarger to trade if needed. Thanks again for all the help you hav e all given. Doug
The current 150 mm El-Nikkor is designated "A" and has the standard 39 mm thread. The previous model is also all-black but has scallops on the aperture-setting ring. This model has a somewhat unusual thread, metric 53 mm dia with 0.75 mm pitch. The lens was sold with a flange designed to use with screws to fasten to a flat board. If you are missing this flange, or want a threaded lensboard or retaining ring, you may have to have it custom machined (e.g., by S.K. Grimes)
Another tip: the front-most portion of the lens screws off, revealing the same 53X0.75 threads, making reverse-mounting of the lens easy (assuming one already has the lens mounted).
Best thing to do is measure it across the diameter.
Neil is correct it may be best to measure. My 150mm Nikkor appears to be current generation based on the photo at the B&H website although it does not have an "A" designation. It came standard with a 50mm to 39mm adaptor so it can be screwed into either type lens board.
I have one of the 150mm's with the large barrel. I did not have a ring either, so I made one, sort of. I mounted the lens on a wooden board for a Beseler using a stiff piece of single weight cardboard cut so it screws around the lens threads, tightly. You would be surprised how strong it is, and way cheap to rebuild. I don't leave the lens on the enlarger with it when I am not there, but it has been mounted with the cardboard ring for several years now, and is just as strong as when first made. I got the idea when I mounted some old barrel lenses for a Speed Graphic that were lacking rings. They are still tight as well. The cardboard will distort somewhat, but as long as the lens barrel is snug against the lens board, it is flat and in place just like any other mounting ring would have it. On one of the Graphic lenses I managed to cut the wooden board so the lens screws directly into the wood, but I did not want to be that tight on the Nikkor threads, so the cardboard ring was used instead. I suppose you could use plastic too, as long as the ring material is a lot softer than the metal lens threads, the metal will just thread its way in and not mess up the lens. Play around a bit and you will be surprised how good a mount you can get.