Steve Bennett (06.26.11, 8:38 PM): Oh, sorry, just to add a tidbit..... you can use a 1.4x or 2.0x extender on the lens, but it knocks out your auto focus, so it becomes a bit more hit and miss. You can find a way to tape over contacts to get the auto focus to work, kinda, but the results are not nearly as consistent. You'll lose a bit in quality and a stop on the 1.4x but can still retain great results, but the quality loss is noticeable with the 2.0x.
Steve Bennett (06.26.11, 8:33 PM): Ditto on the 100-400mm. I shoot primarily wildlife and can't get anyone to buy a child so I can buy the 500mm prime. Love the 100-400.