Questions about Canon 100-400mm L
he 100-400L and 1.4x TC combination should also work with the 5D3 when Canon releases the new firmware in April allowing AF aft f/8. I'm looking forward to it.
It works with low-end bodies, too, if you can live with live view AF (or without AF altogether). I've done that with 7D on occasion; resulting image quality is better than what you get with equivalent cropping, but not much.
Of course, it also works (including AF) with EOS 3 film body, which is what I originally bought it (and the 100-400) for, and with slide film cropping was not an option...