5D III Autofocus

[AE5+]

The autofocus range of the 5DIII is from EV -2 to EV +18, this must be based on the amount of the light entering the lens and the sensitivity of the autofocus sensors.

If it can autofocus at EV -2 at f5.6, can someone explain why it can't autofocus at f8 at EV -1, f11 at EV 0 or f16 at EV +1.

Why the f5.6 restriction, it is so limiting particularly when using extenders.
I have yet to see someone using a f2.8 telephoto, where this wouldn't be an issue.
In the UK the 400mm f2.8 and 500mm f4 cost around £10,000 (16,000 USD).

I have the excellent 400mm f5.6L, but soon as I put on a 1.4x, I have manual focus, and the lens doesn't have IS, so it difficult to use.

I think autofocusing down to f16 lenses should be standard.

 

[Marsu42]

*edited cuz neuroanatomist quoting neuroanatomist has a better explanation!*

 

[sandymandy]

*edited cuz wrong topic!*

 

[Loswr]

This might help, it's excerpted from this article:

Generally, an aperture value is associated with an AF line sensor. The terminology usually used is "f/number-sensitive", e.g, you may have an f/5.6-sensitive line sensor, or an f/2.8-sensitive line sensor. The f/number refers to the maximum aperture of the lens, because AF is performed with the lens wide open (i.e. the aperture you choose for the shot does not matter, only the max aperture of the lens). The use of 'sensitivity' in this context implies that light levels matter, because that's what we think of when we normally use f/numbers. In this case, though, a wider aperture simply means a wider baseline for the rangefinder system is required for that line to function. Personally, I think better terminology might be to use threshold instead of sensitivity, so an f/2.8-threshold line would require an f/2.8 lens to function, and if you mounted an f/4 lens, that sensor line would not operate. An f/5.6-threshold sensor would work with any lens having a max aperture of f/5.6 or wider.

Note that these thresholds are not absolute - a lens with a narrower aperture than the threshold might still work, but at reduced effectiveness, accuracy, and speed. Thus, Canon limits the functionality to the rated aperture for a given AF sensor. However, some third party lenses (e.g. Tamron and Sigma zooms with a max aperture of f/6.3 at the long end) effectively trick the AF system into thinking there's an f/5.6 lens attached. Likewise, although not condoned by Canon, it is possible to use tape to block some of the contacts on a Canon 1.4x extender used with an f/5.6 lens, resulting in the camera attempting to autofocus with an f/8 lens on bodies which are limited to f/5.6. Sometimes, it even works...

The -2 EV is another type of threshold, i.e., an f/2.8 line needs both an f/2.8 aperture baseline and at least -2 EV of light to function.