I've recently had my little Leica IIIf and the 50 Summitar overhauled, and have been using it a bit as my briefcase camera. The 50 feels a little narrow for a normal to me, now.
What other lenses do people like using with a IIIf, and given that there are no framelines in the viewfinder, what viewfinders are you using and how accurate are they? Is it possible to use a 85 or 90 with this camera successfully?
The classic choice would be a Leitz VIOOH: http://www.bhphotovideo.com/bnh/controller/home?A=details&Q=&is=USE&O=productlist&sku=800496460
This sits right over the lens axis so there is no left/right parallax and up/down parallax is corrected if you remember to adjust the wedge provided for the purpose. Voigtländer finders will of course work, as will the Russian turret finder, although these are getting hard to find in good condition. I don't recall the rangefinder base of the IIIF offhand, I always found I could focus a 90 or 135 mm f4 well enough. Avoid the Leitz VIDOM finder, the image is reversed and it will give you a headache!