« on: September 22, 2014, 04:40:45 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.
Hmm, curious. When I rent the 300 f/2.8 II, and use the 1.4x TC, it still seems to focus extremely fast. Faster than the 600/4 with the same TC.
Maybe it's just the design of the 100-400, but that lens doesn't focus fast, period, as far as I can tell. I haven't used the 400/5.6 with the 61pt system, so I can't speak to it.
The 300/2.8 II is arguably Canon's fastest-focusing lens. The focusing group in the 600/4 II is ~25% more massive than that in the 300/2.8 II. The bare 300 II focuses faster than the bare 600 II, so the former will be faster with the same TC. I suspect the 300/2.8 II is just so fast to begin with that you don't notice the AF speed reduction as much.
Here's the relevant bit from Canon:Quote from: Canon DLCAs with previous EF Extenders, usage of Series III EF Extenders lowers AF drive speed to improve AF performance. When Extender EF 1.4X III is used, AF drive speed is reduced by 50%. When Extender EF 2X III is used, AF drive speed is reduced by 75%. This may seem like a drawback, but in reality subject tracking performance remains quite high when Series III Extenders are used with IS II lenses. This is due to improvements in AF precision made possible by the new microcomputer in the extenders.
The 100-400L focuses noticeably slower than the 400/5.6, as well, even though both are 400mm f/5.6.
Actually the 70-200 IS II focuses faster than the 300 IS II. And I can not notice any focusing slowdown on the 300 IS II even with 2x converter. The 70-200 however is slooooow with the 2x. This is my experience with both 5DIII and 1DX, the 1DX is slightly faster with both lenses.
So apparently (wether its true or not) the af speed is supposed to be the same with or without the converters. Af has to move less distance with the converters attached so time to reach focus is about the same. Now getting a lock at f5.6 or f8 is another story