Quote Originally Posted by neuroanatomist View Post
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?