Your DOF at 500mm 7.1 is significatly less than 400mm 5.6. Focal length has a much bigger impact on DOF than aperture.agreed but don't forget about the bokeh, you'll have to work more to blow the background out as it limits your options. 5.6 for me is already difficult enough with low light evening shooting, you really lose it quite fast even at 5.6. Now to cope with a 7.1 lens? no thanks.