TeamSpeed wrote in post #17067642
Why would the mkiii work better with AF over the mkii? Isn't the TC just a pass-through from body to lens?
As with many things, I'm not quite up to speed with latest versions, but I know with the MkII 2X T-Con when it was "reported" to the camera the body would in fact automatically slow down the AF function. Canon described this as being a method to maintain AF accuracy by sacrificing some speed.
Add to that the fact that the loss of two stops of light, and in many cases, reduction of AF sensors down to the single center AF point only, and one should be able to understand why AF performance drops sharply with the addition of a 2X T-Con.
From the T-Con FAQ;
4. Additionally, EF Extenders reduce lens drive speed. The EF 1.4x or 1.4x II reduces lens drive speed by approximately 50~67% depending on the lens in use.
The EF 2x or 2x II reduces lens drive speed by up to approximately 75%.
This speed reduction gives the AF system more time to detect focus. This can be helpful since the depth of focus is reduced with the longer effective focal length and the chance of defocus increases. However, the reduced tracking speed and smaller maximum apertures caused by the use of Extenders can be a disadvantage with fast moving subjects, particularly in low light.