Lens for dog agility / action

[neuroanatomist]

There is some additional info here:


http://www.the-digital-picture.com/C.../Aperture.aspx

And even more here:

http://www.the-digital-picture.com/P...Explained.aspx

And here:

http://www.canon.com/camera-museum/tech/report/2011/09/

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.

I expect we're somehow talking at crossed purposes, here. Fast shutter speeds have nothing to do with AF acquisition speed - phase detect AF is performed with the mirror down and the shutter closed. An an f/2.8 lens set at 5.6 will not necessarily focus faster than an f/5.6 lens set at f/5.6 - depending on the scene, it may actually be slower. If there's extreme defocus, the f/5.6 sensor will be used first, then the f/2.8 sensor - in that case, with an f/5.6 lens the AF would be finished after the f/5.6 step, whereas with an f/2.8 lens (aperture chosen for the shot is irrelevant for AF), the AF system would subsequently refine the focus with the f/2.8 sensor - therefore, the f/5.6 lens would actually focus faster (albeit less accurately).

The bottom line is that true AF speed is a composite of many factors - the specific lens being used (not just the aperture - some f/2.8 lenses focus faster, due to better motors or smaller/lighter focusing elements), the camera (faster Digic means faster calculations, 1-series bodies drive the lens AF motor faster due to the higher voltage), and the scene being focused on (a beat up old 300D with an 18-55mm kit lens will focus faster on a contrasty starburst focus target than 1D IV with 400/2.8 II will focus on an elephant in the fog at twilight).

The next question becomes, if the f/2.8 AF sensor slows down AF, is it actually going to reduce servo performance? The camera will have to work harder to lock onto a bouncing non-constant speed object. Will the increases sensitivity combined with the slower performance mean fewer focus locks, and therefore, fewer shots?

Practically speaking, no. An f/2.8 lens activates both f/5.6 and f/2.8 sensors, and as I stated above, AI Servo actually relies more on the f/5.6 sensors. The 70-200 II is a very fast-focusing lens, at least on my 7D and 5DII.

[Richard Lane]

I expect we're somehow talking at crossed purposes, here. Fast shutter speeds have nothing to do with AF acquisition speed -

Fair enough!

I was trying to infer something else, although poorly, that the latter f/5.6 max aperture lens will focus slower in lower light (due to smaller maximum aperture, less light hitting the sensor) and that this 2-stop loss (4 times less light) as compared to the f/2.8 lens, and that this situation will be worsened when you need faster shutter-speeds, because you are approaching the limits of the lens.

I'm assuming similair lighting conditions and that the user maintains his/her subject on target.

I love your article, by the way!

Rich

[Busted Knuckles]

And even more here:
(a beat up old 300D with an 18-55mm kit lens will focus faster on a contrasty starburst focus target than 1D IV with 400/2.8 II will focus on an elephant in the fog at twilight).

I have no dog in this debate (grin) just thought Neuro's comment was worth a second telling for the humor.

[thekingb]

I'm not a huge cross processing fan, but I really like the second shot of Zuni. Great action shot.