Can any camera keep autofocus active during the actual exposure?
Asked 5/4/2015
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I'm trying to understand whether a camera can continue focusing once the shutter is already open. This came up while thinking about panning: for subjects moving across the frame, focus distance stays fairly constant, but for subjects moving toward or away from the camera, focus would normally need to keep changing during the shot.
On most cameras, my understanding is that focus is effectively locked for the duration of the exposure. Are there any camera designs that can continue autofocus during the exposure itself, and if so, how do they do it?
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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This is possible using Sony SLT technology. It would be possible with cameras that use the imaging-sensor to do autofocus, using on-chip Phase-Detect sensors.
The reason this is not possible on most cameras is that during exposure the Phase-Detect sensor is out of the optical path. On a DSLR, the mirror goes up to expose the sensor and has to be down to let light reach the AF-sensor.
With contrast-detect, which is common on mirrorless and small cameras, you cannot. Even if it is possible to read pixel values without disturbing exposure, the lens needs to be moved in order to know which way the camera has to adjust focus. This is known with Phase-Detect AF.
Sony SLT cameras have a fixed mirror which diverts 30% of light to a Phase-Detect sensor and 70% to the imaging-sensor. Because of this, they can measure focus during exposure and between consecutive shots and know how much and which direction the lens should be moved.
The catch which you can notice when enabling AF-C in high-speed continuous drive (Around 10 FPS based on the particulor model) is that it shoots only wide-open. This occurs because Phase-Detect AF is always done with the lens wide open. It needs a certain variation in light incidence to measure focus which is why we often see that some points are effective to F/2.8, F/4, F/5.6 etc but they stop working when smaller.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
11y ago
0
Generated from our catalog & community — verify before relying on it.
Usually, no. On a traditional DSLR, autofocus during the actual exposure is not possible because the phase-detect AF module is in a separate optical path that is blocked once the mirror flips up.
There are exceptions in designs where AF can remain active while the sensor is being exposed. Sony SLT cameras are the classic example: their fixed translucent mirror continuously sends some light to a dedicated phase-detect AF sensor, so AF can keep working during exposure.
Cameras that autofocus from the imaging sensor itself can also potentially do this if they use on-sensor phase-detect. By contrast, contrast-detect AF generally cannot meaningfully focus during exposure, because it needs to hunt the lens back and forth to determine focus direction.
So the short answer is: on most cameras focus is effectively locked during exposure, but some designs—especially Sony SLT and potentially on-sensor phase-detect systems—can support autofocus while the exposure is in progress.
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