I have the "old style" 40 mm CFE. It has a floating lens element (FLE) at the front where it requires a two stage focussing process. First you focus normally, then look at the focal point and adjust the FLE according to the focus point and recheck the focus. The 50mm CFE has the same design. The 'new' 40mm IF CFE doesn't have the FLE and it's just a plain one stop focussing process. I just like the wider angle the 40mm gives me. Until recently I use to shoot with a rented 50mm all the time. Then one day, the rental place didn't have the 50 and said they'd rent me the 40 for the same price. What a difference! I bought a 40mm the next week. I should of waited a few more weeks for the new 40 mm IF. Drats! Go for the 40mm.