How does DSLR autofocus account for mirror and shutter lag when tracking a moving subject?
Asked 1/26/2015
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On a DSLR, the autofocus system stops receiving light once the mirror flips up for the exposure, and there is also a small delay while the mirror and shutter move. If a subject is running toward the camera at a wide aperture, it seems like focus could be off by the time the photo is actually taken. How do autofocus systems handle this in continuous/tracking AF modes? Do they predict subject motion, and if so, is that typically based on velocity alone or can they also account for changes in speed or direction?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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Most AF systems offer a tracking mode, usually called something like "servo AF" or "continuous AF". In these modes the camera does indeed attempt to calculate the velocity of the object being tracked and account for the time to raise the mirror and open the shutter.
In the top of the line bodies the calculations are quite sophisticated and will take acceleration / deceleration, curved trajectories into account.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
11y ago
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Generated from our catalog & community — verify before relying on it.
Yes. In continuous/tracking AF modes—often called servo AF—the camera predicts where the subject will be at the moment of exposure, since the AF sensor can’t keep measuring once the mirror is up.
At a basic level, the system estimates the subject’s motion and drives focus to where it expects the subject to be after the mirror/shutter delay. In higher-end cameras, the prediction can be more sophisticated than simple constant velocity: they may also model acceleration, deceleration, and even non-linear movement to improve tracking accuracy.
So for a moving subject approaching the camera, the camera is not just focusing on where the subject was when AF last measured it—it is trying to focus ahead to where the subject will be when the image is actually recorded.
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AI11y ago
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