I found an adapter on e-bay that lets me use 39mm screw mount enlarger lenses on my bellows. Here I have my 105mm Nikkor bellows lens and a 135mm El Nikkor enlarging lens. I also can use my 50mm and 75 mm El Nikkor lenses. Why would I want to have lenses of different focal lengths for my bellows? I don't really know but I think it's kind of cool.