I use a Rodenstock 4X with a "liftable" skirt (sounds like my prom date), and it is ideal for 35mm duties as it shows all but the very tips of the four corners of a negative and basically all of a mounted slide. It is well corrected, bright and has no discernable distortion. Similar to a previous poster, I've converted a Konica 40mm f 1.8 "pancake" lens and after rummaging around in the adapters and misc stuff in my drawers, found an old Cokin piece that when attached, hits the focus distance just right and provides a more forgiving plastic surface for touching negatives. It gives just the right magnification for truly determining sharpness, too. I use these for medium format, too. Thay aren't ideal for that, but they've served my needs well enough as the larger negatives are viewable by eye and the loupe is just used to verify details, etc. on them.
