HiFromSusan wrote:
Even with the 100-400, I'd also get a teleconverter. I use a 1.4 with mine, and on a crop sensor (560 mm effective length) it still isn't enough. I'd go for the 2x, even with the 2 stops of light loss you may get.
With a 2X extender the aperture drops to ~f/11 at the long end. Too small for most cameras to auto-focus.