Noise is the key, as far as I can tell a FF sensor usually has at least 1 stop more usable ISO compared to a crop of the same generation. So to get a comparabl

Noise is the key, as far as I can tell a FF sensor usually has at least 1 stop more usable ISO compared to a crop of the same generation. So to get a comparable shutter speed, assuming 1 stop less ISO, you still need that 400mm f/2.8 on crop to match the 600mm f/4 on FF. Humorously, they have the same front element size.