> 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
Hope that helps.
Chuck Westfall Advisor,
ITCG Prof Client Relations Division
Canon U.S.A., Inc.
One Canon Park, Melville, NY 11747