Opemus uses both concave and convex mounted lens boards. In the case of a 50mm or smaller focal length, you screw the lens to the concave side of the board. But if the lens is too large, it will screw only partially (if it does at all) into the board, allowing the light to escape around the thread. It happened to me with a Minolta lens, which is a large model. This doesn’t happen with the 75-90mm lenses, as you screw them to the convex side of the lens board, and in this case the lens’ size doesn’t matter.