Rainbow on Falls, Yosemite Valley, 4:22 PM. (2009 LEICA M9, Auto ISO 160, 1/360 second at f/8, no stinking tripod, 1987 LEICA TELE-ELMARIT-M 90mm f/2.8, 1960 LEITZ SUMMICRON 39mm Polarizer.)
I used a conventional polarizing filter to enhance the rainbow. Polarizers are easy to use on rangefinder cameras: simply look through the filter until you get the effect you want, and put it on your lens oriented the same way. This is easy with LEITZ polarizers, since they have numbers permanently engraved for every 10.º With lesser polarizers, I just look at whatever other lettering is on it, and put it on the lens the same way as I peered through it.
Be careful; if you rotate the camera 90º to vertical, the polarizer will then make rainbow go away if you don't rotate the polarizer, too!