I shoot mainly with 28mm, 50mm, and 200mm prime lenses with my Canon A-1, and with an upcoming purchase of a DSLR I want to continue to shoot with prime lenses, but I've read that a 50mmm SLR lens is not the same as 50mm DSLR lens. What size DSLR lens will I be needing to match what a SLR 50mm is?