WA Tiger wrote in post #14285744
Geez I didn't realise that, is that also without the converters?
Yes, it would be ... But there are no lenses that are over f5.6, so it's not a concern. (Well, actually some third party lenses are f6.3, but they "lie" to the camera and fool it into auto focusing anyway).
You can get teleconverters that don't report to the camera, so it doesn't know it's there and will still try to AF. Or, you can tape up a couple of the electronic contacts on the TC to "hide" it from the camera, and it continue to try to AF. Canon just designs the cameras to not even try, to turn off AF at smaller than f5.6, when it recognizes that condition. They probably just don't want to deal with the complaints, so set up the cameras to not even try below certain light levels.
Your success doing any of the above will vary... depends upon the lighting conditions, subject detail and contrast whether or not AF will be able to lock on. Using these work arounds, you should expect the AF to be slower, more likely to hunt and have trouble locking on.
The new and "better" 61-point AF system of the 5D Mark III (and in the upcoming 1DX) might be a bit more able to keep focusing... It's AF system is rated to be usable in 1 to 2 stops lower light than previous systems. But you'll be on your own and outside Canon's "official" specifications tring to use any combo that's darker than f5.6. There was a bit of a howl from the serious Canonites when the 1DX was announced as not being able to AF at f8, the way all the previous 1D series (and 1V, EOS-3 film cameras) can do. I don't know if this has changed... haven't really been following 1DX news since it's pretty unlikely I'll be buying one. AFAIK, 1DX rollout has been delayed until late June, early July. Canon is being kept pretty busy sorting out the various little glitches in the 5DIII right now.