« on: September 22, 2014, 02:22:19 PM »
They are all cross type, however Canon throttles AF speed at f/5.6 relative to f/4 or f/2.8. Even on the 61pt system, even when using only the center points, AF speed slows when I slap on a 1.4x TC onto my 600/4, vs. just using the 600/4. Additionally, at f/5.6, the chances of "hunting" increase. Canon's AF system caters towards looking for a closer subject first when the scene is out of focus by more than a certain (unspecified) amount. So, if I need to photograph a bird in flight, it's FAR better to use an f/4 lens, which is likely to lock on directly immediately, than f/5.6, which is going to hunt forward first if I'm not already close to focus on the bird.
Maybe the 65pt system changes that, but it looks like the same general firmware as the 5D III and 1D X, so I suspect it'll behave the same.
No, Canon does not throttle AF speed at f/5.6 relative to f/4 or f/2.8. Yes, when you put the 1.4x TC behind your 600/4, AF slows down. But that's not because the combo is f/5.6, it's because of the TC. By design (firmware), a 1.4x TC drops AF speed by 50%, and a 2x TC drops it by 75%. If you put the 1.4x behind the 200mm f/2, you'll have a 280mm f/2.8 lens that activates all 61 AF points including the 5 dual-cross points on the 5DIII/1D X...and still focuses 50% slower.
The 'hunting' you describe also appears to not be specific to the max aperture or the AF points, but rather to lens or TC combo. For example, the 100-400L @ 400/5.6 hunts with a busy background, whereas the 400/5.6L locks on much more effectively in that scenario.