Duct_Taper said:
Since I can't think of an easy way to search for it - what is the fastest in average conditions? And which one is tied with the 100mm f/2 for second?
On average and according to Canon's EU offices, the 300mm f/2.8 IS II is the fastest (hence why Canon market it as such). The 100mm f/2 and 400mm f/2.8 IS II are joint-second. (The 400mm is more-or-less the same as the 300mm, it just has to push a little more glass; the 100mm is fast because it has to push so little glass.)
After that I forget the exact order but roughly it's something like the 135mm f/2, 35mm f/2 IS, 70-200 f/2.8 IS II, 70-200 f/4 IS, 28mm f/2.8 IS, 85mm f/1.8, 16-35 f/4 IS, 200m f/2.8, and 500mm f/4 IS II, as the 'especially fast' focusers. If that's not the exact order it's close to it. After that I really lose track. I want to say the 300mm f/4 IS is in with that lot but I'm less sure on that one, it's been a while.
Though that was as of 2013. Of course there's been a couple of lenses released since then and firmware/manufacturing updates could have adjusted things a little bit too. Even so, we're talking about tiny, tiny fractions of a second difference, and it's not like other lenses are slow. Those are just the ones Canon themselves (at least the EU branch) swear are the fastest. (Under 'normal' light, on pro bodies, with a simple target, etc.) I mean, they don't consider the 50mm f/1.4 to be 'especially' fast, and yet that goes through half its focus range somewhere in the region of 44ms. (44ms is the answer that stuck in my head when I first asked; individual units will be +/-10% and things like age, humidity, and temperature will alter that figure.) For reference, a single frame at 60fps is 16ms, and blinking your eye takes roughly 300ms.
In other words, what Canon considers 'average' is still nearly 7x faster than you can blink your eye. So the difference between that and the very top is minimal. In real world terms you'd feel they're all the same, and if you tried to time them with a stopwatch it'd take you longer to press the 'stop' button than it would for any of the lenses to focus. It's why I don't bother trying to test them precisely myself, I just ask the local Canon reps. (And I trust what they say since the EU offices are generally a little less beholden to the marketing blurb that the American and Japanese offices are.)