A 500mm f/4.0 makes more sense to me as it is still hand holdable for a few shots like the 400mm and has the added extra 100mm to make it a real "long" and fast lens. With the improved low light camera bodies the f/4.0 is not that bad and the bokeh is still excellent. I may well be buying an RF 500mm if they make it.I wonder if they will produce a 500mm lens again. Are there really enough people who want 500mm, because 400mm are too wide and 600mm are too narrow for them. 500mm would only be interestng for more people if there was a f/2.8 version, but that one would be very heavy. I expect the 200-500mm f/4 to be a good alternative to a 500mm prime.