I don't think I'm missing the point, the point is the lens motor is powered neither at 11.1 V nor at 7.2 V. It is powered at a voltage which I don't know, but which is undoubtedly the same whichever body you are using whith whatever battery voltage/number of batteries you are using. The lens is NOT powered directly from the battery, but from a circuit which delivers constant voltage whichever body/battery combo you are using.
If the lens motor was receiving the same power from a 1D X and a 7D, then the AF speed for the lens would be the same with both bodies. But it's not, it's faster with the 1D X.
The lens receives both a power line and signal lines from the camera 'Quality' of signals are different for sure, coming from different camera subsystems running different AF routines. But if a lens is designed to be powered at a certain voltage, differet bodies should deliver the same DC output to the lens in this respect, regardless of the main battery specs. So, as far as AF
is concerned, it is not only possible, but even reasonable that, although receiving the same power from the 1Dx and 7D, a lens focuses faster on a 1Dx. Rather, I would be surprised of the opposite. But I can't explain myself how focusing should be faster under the test conditions you described previously when focusing manually
by wire, at least that's what I understand you did with your 85mm L II, please correct me if I misunderstood you. Don't get me wrong, I'm not saying I don't believe you, and thanks again for reporting, there's just something I'm missing...
Could it be firmware trickery on the part of Canon, the 7D delivers the same power as the 1D X, but the 7D just tells the lens to AF slower? Possible, I suppose...but that seems a little too cynical, even for me.
There are a couple of interesting articles by Roger Cicala from LensRentals which, although dealing with AF accuracy and precision, may partly justify also the improved AF speed.http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-ii-1-vs-2-and-old-vs-newhttp://www.lensrentals.com/blog/2012/08/autofocus-reality-part-3b-canon-cameras#more-8458
Something tells me you already know these articles.
It appears to be a matter of old vs. new, 5DII/7D vs. 1DX/5DIII/6D, maybe both hardware and software. I truly believe it's not a matter of battery voltage, rather it may be the camera subsystems which drive the lens' AF.
So, paraphrasing your words, we may suppose that the 7D delivers the same power as the 1D X, but the 1D X just tells the lens to AF faster
. Put this way, the statement turns from cynical to logical.
The battery does help a camera body autofocus an EF lens faster.
From Chuck Westfall… “In addition, the EOS-1D X achieves a higher lens motor drive speed with select L-series USM telephoto lenses than the 5D Mark III because of the 1D X’s more powerful battery pack.”
Exactly, power, i.e. V x A = W.
1 x LP-E4N is 11.1 V x 2450 mAh = 27.195 W
2 x LP-E6 is 7.2 V x 1600 mAh x 2 = 23.040 W
27 W vs. 23 W, not much of a difference, but it's there.
mAh is a measure of electrical charge and not max current output
Well, English is not my native language, so I apologize if I'm not using the correct technical terms for physical quantities, I'm translating those terms straight from my language, so forgive my eventual lack of precision. But let me say 'electrical charge' is a totally different thing, and its SI unit is the Coulomb, not A or Ah. I guess you are referring to the battery capacity, or the total amount of energy it can store, so your statement should be rephrased: 'mAh is a measure of the amount of electrical charge and not max current output
'. Fine. Still, I don't understand where this conflicts with what I wrote.
Sure, a Li-ion battery rated 7.2 V @ 1600 mAh could deliver currents much higher than the nominal current (provided the chip inside allows it, I think it doesn't), but at a much lower voltage. If you short-circuit a non-chipped batt, the current flows massively, the batt overheats, the chemicals inside alter and you kill it. This is why the V + mAh spec of a battery are termed nominal voltage and nominal capacity. Someone better skilled than me in physics please chime in and explain better, I'm sure there is. But we're going a bit off-topic here, I think.