I am relatively new to enlarging. I've been working with a Beseler 23C II, enlarging 6x6 with an El-Nikkor 80/5.6 lens.
Recently I tried 6x9, but found that the corners are vignetting. After reading a bit, I gathered that I needed a longer lens, so I bought an El-Nikkor 135mm lens.
When I attached this to my enlarger, I couldn't get the image to focus. After more research, it turns out I need a lens board with extension.
So now I've got a board with about 20mm of extension "cone", but it is still not enough! Don't know what's up with this lens. It
On a whim, I attached my 80mm lens to the extended board, and now it seems to have better coverage. To my surprise, it still has room to focus as well.