regarding the different focal length enlarger lenses... if you want to reprint an image and the lens you used is not available you'll either be back at square one (test print) or need to do some mathematics to calculate the appropriate time. I always use 50mm for 35mm, 80mm for MF and 135mm for 4x5, except when I print postcards, then I might use a longer lens to get more working room between lens and baseboard.