Talley wrote in post #16767426
The 6D Center cross is a 5.6 cross type in a + pattern. There is an additional non cross single line that overlays the cross in a - shape that is 2.8 sensitive. It captures vertical lines (because it is horizontal it sees anything vertical.
So again. That 2.8 sensitive horizontal line in the center is very sensitive and very good but it's not a cross type. The center cross is only 5.6 sensitive. But together they work good. If you don't use 2.8 or faster glass then you lose that capability of the 2.8 but you still benefit from a + type cross sensor and another - overlayed on top of it.
Now the 5D3
It does have multiple cross type dual AF in the center but they are not dual 2.8 cross type. The 5 vertical center points have one 2.8 cross in a X format and a 5.6 sensitive overlayed on top in a + format creating a dual cross. It's outer points surrounding the center 5 are regular 5.6 cross as found on the outer points of the 7D and the further points in blue are F4 vertical sensitive and 5.6 horizontal sensitive cross type and the furthest outer points are just regular horizontal types as found on the 6D outer points.
I'm not sure but I believe the 7D, 60D both have actual dual 2.8 cross type centers not found in either 6D or the 5D3, these two have a different method as mentioned above.
Somebody check me on this.
The 6D Center cross is a 5.6 cross type in a + pattern. There is an additional non cross single line that overlays the cross in a - shape that is 2.8 sensitive. It captures vertical lines (because it is horizontal it sees anything vertical.
So again. That 2.8 sensitive horizontal line in the center is very sensitive and very good but it's not a cross type. The center cross is only 5.6 sensitive. But together they work good. If you don't use 2.8 or faster glass then you lose that capability of the 2.8 but you still benefit from a + type cross sensor and another - overlayed on top of it.
Now the 5D3
It does have multiple cross type dual AF in the center but they are not dual 2.8 cross type. The 5 vertical center points have one 2.8 cross in a X format and a 5.6 sensitive overlayed on top in a + format creating a dual cross. It's outer points surrounding the center 5 are regular 5.6 cross as found on the outer points of the 7D and the further points in blue are F4 vertical sensitive and 5.6 horizontal sensitive cross type and the furthest outer points are just regular horizontal types as found on the 6D outer points.
I'm not sure but I believe the 7D, 60D both have actual dual 2.8 cross type centers not found in either 6D or the 5D3, these two have a different method as mentioned above.
Somebody check me on this.
Here's the 6D's AF sensor:
h14nha wrote in post #16768093
It also helps they are dark coloured birds against clean uncluttered backgrounds.
It also helps they are dark coloured birds against clean uncluttered backgrounds.
The 6D has a lower keeper rate in AI Servo than some other bodies, but if you have patience and skill it's enough to get great results, the photographer is more important than the autofocus system. Still, if you're a bird photographer, choosing a 6D is a questionable choice. 
I find the slow frame rate compared to say the 7D a bigger issue than the AF system. But the 5D3 is not hugely better in this area.
