But, I mean, how do you know the f/2.8 sensor isn't working with AI Servo on those cameras? What sort of indication are you getting?
I use Servo AF a lot for black dogs running towards the camera and also for BIF's, motorcycles coming towards the camera and the like.
There are two reasons I do not think the high precision sensors are used during Servo AF. The first reason is just my conjecture but the second reason is based upon real world usage:
1) Servo AF takes multiple distance measurements during tracking and uses these measurements to determine at what rate the subject is approaching the camera and whether the subject is accelerating or slowing. It uses those measurements to guess where the subject will be during the next time the shutter open. It is my conjecture that the errors involved in guessing where the subject will be during the next exposure are large enough that they would dwarf any additional accuracy that may be squeezed from the high precision sensor. Additionally, the high precision sensors are not as reliable in getting a reading as the regular sensors (even on static objects, as my testing shows, let alone moving ones) and Servo AF requires a constant stream of good data to provide reliable tracking for a subject that may be constantly changing the rate of closure. In other words, it would be more important to have reliably good readings of a slightly lower accuracy than to have readings that varied from excellent to poor. Remember, the AF system has no way to tell the difference between a high precision reading outlier or a change in speed or the angle of closure of the subject.
2) I have used my 70-200 f4 IS lens extensively in Servo AF mode as well as my 300mm f2.8 and, recently, the new 70-200 f2.8 IS II. I use them on the same types of subjects in the same conditions using the same body and the same AF settings. A lot of my Servo AF usage is of dogs running toqwards the camera. If f2.8 lenses had an accuracy advantage in Servo AF mode it should show up in my images. But it doesn't. So, if there is
any benefit, it is so small as to being meaningless. Actually, my sense is that I get a slightly lower hit rate with my top quality f2.8 lenses than with the 70-200 f4 IS which leads me to think larger aperture lenses may be somewhat
disadvantaged slightly in terms of reliable Servo AF tracking (and also in terms of AF lock in One Shot AF mode). But it is certainly no better in my experience.
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Mike Mullen
I too have noticed I'm getting consistently unreliable autofocus with fast lenses (keeper rate 20% or less), but when I switch to my F4.5 lenses the autofocus is spot on and my keeper rate shoots up to about 80%.
