I ran in this problem years ago, too, "big" enlarging lenses cannot be fitted into the reversed board, because of its "bell" shape. The solution is to use a "plan' lens board, and then, the lens is closer enough to get proper focus. I have to check it, but I believe I recycled a Durst lens board for this on my Opemus. If you have a retaining ring for M39, your plan lens board don't even need to be threaded.
Originally Posted by octofish