Will someone please help me understand this?

Center: cross-type at f/5.6; vertical line-sensitive at f/2.8.

So on my 6d the center point becomes a cross-type at 5.6 but when I am using my 70-200 at 2.8 its only vertical line sensitive?

How would this affect shooting a moving child, dog, or bird? Would I be better off opening my 70-200 up to 5.6 to get better accuracy assuming I was using center point only. Or for that matter how dose it affect accuracy on a static object?

I thought I had my head wrapped around this but now I am just a little confused again:o

Thanks for any replies in advance

Auto focus is always done with the lens wide open. If your lens is 2.8 or faster no matter what aperture you shoot at it will still make use of the super sensitive feature.

It is still a cross type focus point it is just that the vertical line is even more sensitive.

^ Bingo!

I may be mis-quoting, but the cross type will work with any lens 5.6 or faster (IE 4.0, 2.8, 1.4, 1.2)
Therefore you will not have cross-type AF when using a telephoto 5.6 lens with a 1.4x or 2.0x converter.
Because the f number is over 5.6
It will be more sensitive, work "better" with a 2.8 or faster lens. (2.8, 1.4, 1.2)

If I am wrong then please correct me.

InfiniteDivide wrote in post #16620002
...Therefore you will not have cross-type AF when using a telephoto 5.6 lens with a 1.4x or 2.0x converter.

If I am wrong then please correct me.

You won't have any AF at all with a f5.6 lens + 2X converter(without some trickery) as that would be f11.

rrblint wrote in post #16620027
You won't have any AF at all with a f5.6 lens + 2X converter(without some trickery) as that would be f11.

Then I stand corrected, thank you.
I knew the converters raise the f stop, so I assumed that would be when (after 5.6) you lose the cross type AF
Then it would apply to any lens slower than 5.6, no cross type?

InfiniteDivide wrote in post #16620037
Then I stand corrected, thank you.
I knew the converters raise the f stop, so I assumed that would be when (after 5.6) you lose the cross type AF
Then it would apply to any lens slower than 5.6, no cross type?

It's just that it won't AF at all after f/8. You'll have cross type down to f/5.6.

Thank you for responding folks. I just plumb got myself confused by over thinking..............​.I think:o:o

So that is why the 5d3 will still af with a 400L and a 1.4x on it then, because it has at least one point that is good to f8 right?

Have a look at page 32 in these papers about the 5D Mark II. The center point seems to work in a similar way to the 6D.

When offering f/8 focusing capability, the 1DX can use only the five points in the center, and only in single line sensitive mode. I presume it's the same for the 5D Mark III.