Having read several posts discussing the pros and cons of using AI Servo AF versus One-Shot AF for stationary subjects, I thought I'd ask the one guy who could best address this issue for us.
He was kind enought to allow me to post his reply to my questions here:
There are no differences in focusing speed, focusing accuracy, or focusing point selection algorithms between One-Shot AF and AI Servo AF with EOS DIgital SLR cameras, period.
The basic difference between them is that One-Shot AF locks focus as soon as it is complete, whereas AI Servo AF continues to track focus as long as it is active. This is why One-Shot AF is recommended for stationary subjects, while AI Servo AF is recommended for most types of moving subjects, especially those that move towards or away from the camera as opposed to lateral movement across the frame.
There are other differences between One-Shot AF and AI Servo AF that can affect some kinds of photography:
1) AI Servo AF allows photographers to release the shutter at will, regardless of whether focusing has been completed or not. This is intentional, in order to allow the photographer to prioritize capturing the peak moment regardless of focusing status. The trade-off is the fact that there is no guarantee that the focus will be sharp on a stationary subject in AI Servo AF, especially during handheld photography at close range with shallow depth of field. Under these specific conditions (one more time for emphasis, I am saying Stationary Subject, handheld photography at close range with shallow depth of field), One-Shot AF is a more reliable focusing method because it locks focus while AI Servo does not.
2) As light levels diminish, eventually AI Servo AF will cease to function before One-Shot AF does. This is because One-Shot AF allows a longer sampling period for AF measurement in low light than AI Servo does. (The AF measurement sampling period is analogous to a shutter speed for the AF sensor. The longer the sampling period, the greater the sensitivity.) Remember that the AF sensor in the camera has a low light threshold, typically EV -1 or -2 depending on the camera; this figure is quoted specifically for the center AF point with One-Shot AF. It's usually about 2 stops less than than with AI Servo AF, and even lower with off-center focusing points. Therefore, if maximum sensitivity for AF in low light is your priority, we strongly recommend One-Shot AF with the center focusing point.
Going back to point 1, current professional EOS models like the 1D C, 1D X and 5D Mark III give photographers more control over shutter release priority in AI Servo AF than older models. You'll notice that there are menu settings in the AF menu section for 'AI Servo 1st Image Priority' and 'AI Servo 2nd Image Priority.' These settings let you control how long the camera waits before releasing the shutter in AI Servo, which is better than older cameras like the 1D Mark IV or 5D Mark II. But it still lets the camera shoot when it is out of focus in AI Servo AF if you insist. In other words, shutter release in AI Servo AF is always a matter of "when," it is never a matter of "if" the subject is in focus.
The bottom line is simply this: AI Servo AF is *not* equivalent to One-Shot AF for stationary subjects in terms of shutter release priority, especially for handheld shots with shallow depth of field, and we never claimed that it was. That's why we offer both focusing modes. This doesn't mean that AI Servo *can't* get it right. It means that One-Shot AF is more reliable under these specific conditions.
Hope that helps.
Advisor, Technical Information
ITCG Prof Client Relations Division
Canon U.S.A., Inc.
One Canon Park, Melville, NY 11747