Bill: There is not a robust rental market for these lenses, as far as I know. Others may correct me if I am wrong. One option would be to buy from a seller with a generous return policy. KEH comes to mind, or you could buy from an RFF'er with the understanding that you could return the lens if it was not to your liking. I would vote for the 50/1.5 Nockton or the 40/1.4 from Cosina/Voitlander. If you live in a major market (NY-LA-SFO-BOS etc.) you may get a retailer to lend you a used lens to shoot a couple of rolls with (sort of a leave-your-drivers-license deal). The "will not break the bank" part of your subject line, is of course, very subjective. If you have a Summicron, you already have a great low-light lens. If you routinely need to go below 1/30, I would consider a) a small bean-bag on which to rest the camera, b) a monopod or c) a table-top tripod (Bogen makes a nice one with a ball head that you can also brace manually against a wall or door-jamb). I know that none of these are as sexy as a new-to-you lens, but they will get the job done. I'd echo what the poster above said about focus issues at f2. Any M camera with a properly adjusted rangefinder should be able to accurately and consistently focus a 50/2 wide open and close up.