Wouldn't it be easy enough to modify a cheap extension tube (like this one on eBay) so that a osciliscope could be hooked up to the Vbat and A_GND contact to see exactly what voltage is driving the AF system for a given camera?
That might not tell you what's going on downstream between the AF electronics and the USM motor, but it might help to support the higher-voltage/faster-AF theory. If the voltages for a 7.4V-based camera and an 11.2V-based camera are the same, then that theory can be put to bed.
Actually, after reading the nth post claiming "a higher voltage may likely drive the AF motor faster", "maybe Canon will develop higher voltage batteries..." "it seems there's still someone who doesn't get that a higher voltage..." and things like those, I gave up replying. But look at the figure above: Vbat
is the power line which feeds the lens' motors, and it's +6/-6 Volts in any camera
is the power line that feeds the logic, and it's +5.5/-5.5 Volts in any camera
. You can check it yourself, no need for an oscilloscope, just use a Voltmeter on your camera, the red terminal on Vbat and the black one on A_GND. Note that the large contact A_GND on the lens corresponds to 2 contacts on the camera, pick any of them: you'll read 6 Volts.
By the way, Chuck Westfall NEVER said the 1D X focuses faster than the 5D III because the battery has a higher voltage, he said it's because it's more poweful, and, indeed, it is. Roughly 4 W more than dual LP-E6.
I think I'll start another thread on this matter in the near future, it seems so many are interested, and it also seems there's a lot of misconception regarding this.