Appreciate the input.
The more I think about it, seems I would probably be best served by keeping and using the Canon 100-400 and enlarge/crop if necessary.
Hey Bob,
Did you ever get the Osa Mauser?
As far as AF goes with a 1.4x TC you may want to research that as some of the aftermarket TC's like the Kenko or Tammy may not report and so your AF will work. You can also tape the left three pins so your your 100-400 will AF with the 1.4x. I have done this with the 100-400 several times. However this is something you want to test with your lens in advance as the AF can be slower.
Instead of just focusing on FL I'd suggest you give aperture some thought as well. The 100-400 is already f5.6 @ 400mm and you loose another stop with the TC.
You may want to consider looking for a used Canon 300 f2.8. You could use it then re-sell it after your trip and have use of the lens for little more than the cost of freight. This is far more cost effective than renting if the purchase is an option. This way you would have 300 2.8 and 420 f4IS with the TC at a fraction of the cost of the 500 f4IS.
Either way the 100-400L is a great piece of glass for your trip that offers a lot of flexibility.
http://www.dgrin.com/showthread.php?t=29503 IMO, dropping a bunch of cash on a 300/2.8 w/1.4x, to gain only 20mm of focal length, and one stop of light, is a waste of money. The 100-400 you already own, will work just fine, and if you need another stop, crank up your ISO. The only meaningful step up, would be to a 400/2.8 or a 500/4. Both of which are very heavy, and very expensive.
I'd also forget about trying to use a teleconverter on a f5.6 lens for wildlife. Under ideal conditions, sure it will work, albeit slowly. Shooting in the morning/evening, on moving game(and most African game is always moving), it will give you fits. If you're going to be in the bushveld, your auto-focus will be challenged enough, trying to focus on a subject that doesn't contrast greatly with the background, in addition to being obscured by grass/brush. Couple that with trying to make it work at f8(f/5.6 lens w/1.4x), and you'll frustrate yourself.
Unless you're cool with sinking a ton of money on a long prime, just practice with your 100-400 off of improvised rests, and have a ball! That's JMO, of course.
Jeff
