By using back-button focus you don't have to switch from "AI servo" when photographing stationary subjects. I tend to do this anyway since I have the feeling it's more accurate even when I can disengage the focusing independently of the shutter shutter button.
Is the "One shot" mode more accurate than "AI servo" even if one uses back-button focus correctly?
It depends on what you mean by more accurate. The same algorithms are used in either AI Servo or One Shot to get an initial focus on what is in the viewfinder at the selected areas of AF sensitivity.¹ What happens after that may affect how well your subject is in focus in either mode.
With One Shot the camera stops all AF focus activity once focus lock is achieved until after you either:
If your subject moves between the time you lock focus and take the photo you are not going to get accurate focus.
On the other hand, AI Servo continuously checks focus and can occasionally "hunt" a little between the time you activate AF and you press the shutter all the way to take the photo. Whether the camera takes the photo immediately or waits until it can confirm focus to fire the shutter depends on your release priority settings. So if the camera is "hunting" when you fire the shutter you can potentially get an inaccurately focused picture.
If the release priority for AI Servo 1st Image Priority and AI Servo 2nd Image Priority under the AF2 menu are set to focus priority there should be no difference in accuracy between AI Servo and One Shot. This assumes you properly told the camera where you wanted it to focus and the subject hasn't moved since focus lock was achieved. If release priority is set to equal priority or release priority then AI Servo can be less accurate since you have told the camera to hurry up and take the picture!
¹Although the same algorithms are used, Chuck Westfall, Canon USA's chief technical advisor for decades, has told us that the sampling period for One Shot AF is longer than the sampling period for AI Servo AF. This means that in bright light AI Servo should be slightly faster, but in low light One Shot will function at dimmer light levels than AI Servo can. The longer sampling period is the rough equivalent of a longer shutter time in terms of how much signal is divided by the constant noise produced by the PDAF sensor. For a more in depth discussion of the low light differences between One Shot and AI Servo, including two detailed quotes from Chuck Westfall, please see: Is there any reason you would use one-shot focus over AI-Servo?
At least for Canon cameras, this excelent explanation from Canon's Chuck Westfall should answer perfectly your question (AI Servo AF Versus One-Shot AF For Stationary Subjects):
Hi, Steve:
> 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.
Chuck Westfall Advisor, Technical Information ITCG Prof Client Relations Division Canon U.S.A., Inc. One Canon Park, Melville, NY 11747
