Quote Originally Posted by Richard Lane View Post
It is true however, that the more light the maximum aperture lens lets in, the faster the AF sensor will acquire focus.
It's more about contrast than amount of light (although the two are related). But a high contrast transition at an f/5.6 AF point will lock on faster than a low contrast transition at an f/2.8 point. The AF sensor lines have a minimum threshold at which they operate, and as long as the intensity of light is over that threshold and there is sufficient contrast, there is no speed difference with more light. As I stated, a dual-sensitive point will actually use the f/5.6 part to 'pre-focus' because the f/5.6 line(s) are faster than the f/2.8 line(s).

As Canon has stated, "Using f/2.8 light rays is advantageous for two reasons. Because the aperture is larger and optically brighter, it is possible to detect the focus in darker conditions. And because the base line used in the triangulation principle is longer, more accurate focus measurements are possible. The disadvantage is that it may take some time to search and find the focus when the subject is well out of focus because the detection range at f/2.8 is narrow."

The 'darker conditions' apply to light levels down near the low end of the AF sensor sensitivity (-0.5 EV to -2 EV), where more light can push it over the threshold.

Quote Originally Posted by HDNitehawk View Post
Interesting. Why do the supertele's focus slower when you put a 1.4x or 2x extender on? Decrease in aperture, increase in length or addition of hardware?
It's implemented due to the camera firmware (non-reporting TCs do not slow down AF). Canon has stated the speed reduction is important for greater accuracy.