Leica suggests specific combinations of Elpro lenses (which are achromatic doublets) and focal length lenses. I have an Elpro that I don't use often, but the few photos I took with a 180mm and Elpro last spring on Velvia 50 showed no sign of the softness that shows up away from center with many other supplementary lenses I've seen. Minolta made doublet close up lenses as mentioned earlier that you might find used, and some filter makers also produce them, although I forget whom. In addition to the Nikon and Canon, I'd check on B+W, Heliopan, and perhaps Hoya for doublets.
Originally Posted by Claire Senft