When is single autofocus better than continuous autofocus?
Asked 11/24/2013
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2 answers
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If I usually move the AF point instead of focus-and-recompose, is there still any reason to use single autofocus (AF-S) instead of continuous autofocus (AF-C/servo)? In what situations is continuous AF less suitable, and when does single AF make more sense?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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Reliability is the true reason. When a camera focuses continuously, it keeps measuring focus and readjusting. In a perfect implementation it would lock focus and follow your subject perfectly but that does not happen. In practice, cameras spend a good percentage of the time catching up to a subject's motion, so your subject may not be exactly in focus at the precise moment you want to take the shot. Higher-end cameras do better but still, none is perfect 100% of the time.
Another reason is battery-life. Continuously focusing drains the battery quicker than not. There are some cameras equipped with an AF-A which is designed to do AF-S first and then switch to AF-C at the point subject motion is detected. That is there more to save battery life and make things more predictable but is not perfect either. Particularly if you use automatic focus-point selection which means that some active AF-points will be outside of your intended subject and the camera can see movement on one such point and shift focus to another subject, potentially causing you to miss a shot.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
12y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. Continuous AF is best for moving subjects, but single AF can still be the better choice in several situations.
First, AF-C is constantly re-measuring and adjusting focus. In real use, it may be “chasing” the subject, so focus may not be exactly where you want it at the instant of exposure. AF-S is often more predictable for stationary subjects because it locks focus once acquired.
Second, AF-S is useful when you want to lock focus and take several frames without the camera trying to refocus as composition or viewfinder conditions change between shots. This can matter especially with back-button focus setups, depending on how your camera is configured.
Third, continuous AF uses more battery power because the camera keeps driving the focus system.
So a practical rule is:
- use AF-S for still subjects, deliberate compositions, and when you want focus locked
- use AF-C for subjects that are moving or likely to move
Higher-end cameras track better, but no continuous AF system is perfect all the time.
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