Lens for dog agility / action

[Richard Lane]

Richard, this is the first I've heard of the maximum aperture affecting the focus speed. I know there is the f/2.8 focus sensor for higher precision, but that an f/4 lens would focus half the speed of an f/2.8, or and f/5.6 being quarter speed, I've certainly never heard of that.

Sorry, about the confusion. I combined my thoughts of AF f/2.8 sensor speed and aperture focusing speed. To clarify, I have edited my original post and I have changed the words "focusing speed" to "lens speed." The f/2.8 is definitely much faster, but my two f-stop, 4 times faster focusing example is overstaing the difference. I meant for my example to reflect "lens speed" as in the faster shutter-speed that may be obtained. It is true however, that the more light the maximum aperture lens lets in, the faster the AF sensor will acquire focus.

Rich