Lens for dog agility / action

[Richard Lane]

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).

I was referring to the concept that twice as much light from the larger aperture was being directed towards the AF Sensor, which will increase AF acquisition speed, and why an f/2.8 lens set at 5.6 will focus faster than a f/5.6 lens set at 5.6. Of course this will be accentuated in lower light conditions and/or at faster shutter-speeds in which the aperture will be open for a shorter duration.

Rich