The wide aperture affects AF speed and AF point precision and capability. Remember, particularly in the 61pt AF system, there are f/2.8 double cross type points, f/4 cross type points, f/5.6 line points, and the center expansion f/8 points. With an f/4 lens, you ALWAYS AF at f/4, no matter what you stop down to for actual shooting. The extra stop of light allows the AF system to operate more quickly and more accurately. When f/4 AF points are used, they tend to be more precise than f/5.6 points, which need larger pixels in order to sense as well as f/4 pixels.
The point of an f/4 lens isn't that you always shoot wide open (although in the evening, it isn't uncommon...I tend to be around f/8 aperture for shooting during daytime, and f/4-5.6 for shooting around sunset, for wildlife.) It's that you ALWAYS AF wide open (by design.) And yes, with an f/4 lens, when you slap on a 1.4x TC, you still AF at f/5.6, which is still better than AF at f/8, no question.
Jrista, thank you.
I actually completely forgot about the difference between f4 AF points and f5.6 ones. The high precision f2.8 AF points are referenced so much I guess I clumped all the others into the same group. (After reading multiple articles detailing all the AF points, and watching the entire B&H Canon AF seminar [ ww.youtube.com/watch?v=iAx86nblZ2g ][great video BTW], you would think that someone would remember something like that. I guess a guy can only fill his head with so much.)
That changes my perception of the TCs quite a bit. I've been assuming that you get "worse AF" when using a TC because of a combination of optical performance and some kind of interference in the circuitry, if on the other hand it's actually just due to the change in the type of AF points used, then the 600f4+1.4xTC is actually going to AF exactly the same as the 800f5.6. I was assuming the latter would have an advantage.
Indeed that does make a very good case for the 600f4, and makes the 300f2.8 more appealing now that I know it's not some mystical interference from the teleconverter making AF worse at 600mm.
IMO a teleconverter, no mater how good it is ( and how sharp the lens is ) degrades the image. And that also has an effect on AF accuracy and speed. As well as the loss of 1 or two stops of light.
I would love to see a auto-focus test using the 600 II with a 1.4 against the bare 800 F/5.6 lens. With the same camera it is my guess! The 800 would win. Not only being faster ( though very slightly ) but more important more accurate. Looking at the MTF charts of the two lenses it would be a very close race for sure.
Look at this link 1.4 tc III on the 200 f/2.8 lens. http://slrgear.com/reviews/showproduct.php/product/1367/cat/62
And here for the 2.0tc III: http://slrgear.com/reviews/showproduct.php/product/1366/cat/62
Looking at that data. Pretty much clears things up for me when using a t.c. I wish these guys would test the 600 II and 500 II using T.C.'s.
But even if that is true. I would much rather have a 600 II with a teleconverter any day...