Lens for dog agility / action

[HDNitehawk]

I'm puzzled by Rich's statements about slower focusing, too. The aperture doesn't directly affect focus speed, AFAIK. While an f/2.8 lens provides more precise AF, that precision does come at a slight cost in speed - that's why the majority of the data used to drive the AI Servo calculations actually come from the f/5.6 sensor lines - they have a faster response time (similarly, with the high-precision AF points, the AF system uses the co-located f/5.6 line(s) for a 'coarse focus' then refines that with the f/2.8 line(s).

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?

Has any one clocked the focus speed of a variable aperture zoom to see if the speed increases or decreases from one end to the other?

[neuroanatomist]

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.

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.