The only one I can think of is recomposition - achieving focus and then recomposing. Are there any others? The convenience of tracking your subject seems very, very convenient - moreso than the lock confirmation.
Answer
There are some other considerations, depending on the camera in question. I'll address it from the point of view of Canon EOS cameras (which is where the nomenclature AI Servo originated).
Situations in which One Shot will perform better than AI Servo for Canon EOS DSLRs:
Low light conditions. Per Chuck Westfall, the head of Canon USA's Professional Client Relations Division and former Canon Technical Advisor:
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.
When confirming focus is critical and the subject is static. Again from Chuck Westfall:
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.
When the shutter button is pressed all the way in AI Servo the camera fires whether focus has been achieved or not.
When the shutter button is pressed all the way in One Shot the camera will not fire until focus has been confirmed. If the shutter has already been half pressed and focus has been confirmed, the camera will fire immediately, just as in AI Servo. But if the shutter is pressed all the way and AF has not already been confirmed, the camera will wait as long as it takes, from just a few microseconds all the way to eternity, before it will release the shutter.
Some of Canon's more recent advanced cameras include options for the user to fine-tune the balance between AF confirmation and fast shutter release. It can even be set differently for the first shot in a burst and the subsequent frames in that same burst.
Chuck, as well as other Canon reps, have made similar statements elsewhere, but here's a link to the most succinct version of it.
There's also a lot of misinformation around the internet (imagine that) that says differently, but Chuck has been the go-to guy for technical information at Canon USA since the EOS system was introduced in 1987. Some of the misinformation still going around is based on how models current almost three decades ago behaved rather than how the current lineup works.
The following statements are true of all of Canon's current EOS lineup, and pretty much any model released in the past 10 years:
- In all but very dim lighting conditions AI Servo and One Shot are equally accurate.
- All other settings being the same in both cases, the camera focuses on the same exact thing in both AF modes.
- Single points are the same size with AI Servo or One Shot.
- Expanded points are the same size with AI Servo or One Shot.
- AF areas are the same size with AI Servo or One Shot.
- Automatic point selection works exactly the same way with AI Servo or One Shot.
The only difference between AI Servo and One Shot other than those referenced above is that AI Servo continues to track the subject in the way that it has been instructed until the shutter is released (picture is taken) while One Shot stops tracking and holds the same focus distance between the time it confirms AF has been achieved in the way that it has been instructed until the shutter is released (picture is taken).
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