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The origin of this question is related to panning. I was trying to explain why panning is more effective for subjects moving across your FOV at right angles, vs. a subject whose distance to the camera is changing -- namely, once an exposure starts, focusing is locked for the duration of the shot (as far as I know).

It occurred to me, though, that there are a couple possibilities for focus-following during exposure (and probably some I haven't thought of), including:

  • AI focus (Canon) -- as advertised, this is "predictive" autofocus, so I'm not sure why it couldn't keep predicting during an exposure
  • Live-view focusing -- probably more difficult, technically, because the camera would have to switch focusing methods as soon as the exposure begins and pick up the tracked subject. Might be easier with something like Sony's translucent mirror.

In any event, I'm unaware of anyone offering an actual feature like this today -- is that right?

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    \$\begingroup\$ Are you looking to focus during a single exposure? Or looking to focus during video, or multiple exposures? Changing focus during a long exposure is sure to result in a totally blurred image. Tracking focus during an ongoing, long exposure doesn't have any real value. As for focusing during say video...Canon has DPAF which will do that, and other manufacturers have similar things (although usually done with CDAF). \$\endgroup\$
    – jrista
    May 4, 2015 at 17:10
  • \$\begingroup\$ I'm looking for focus during an exposure, as in a panning shot where the subject's distance varies during the exposure. \$\endgroup\$
    – D. Lambert
    May 4, 2015 at 17:32
  • \$\begingroup\$ Wouldn't autofocus during exposure be kind of moot as the exposure time would be an order(s) of magnitude less than the time needed by the camera to move the lens? \$\endgroup\$
    – Peter M
    May 4, 2015 at 17:57
  • \$\begingroup\$ @PeterM - I was thinking that if implemented anything like Canon's AI focus, the rate of change would be close to constant -- not completely unlike star tracking (though obviously quicker). If you're tracking a subject moving at a known rate of speed, lens movement might start at the same speed right off the bat. \$\endgroup\$
    – D. Lambert
    May 4, 2015 at 18:37
  • \$\begingroup\$ OK .. Now I can see constant focus tracking as feature. However I can see that it probably would fail if the object approaches you and then recedes from you during the shot. EG standing on the side of the road and panning a car from left to right as it passes right in front of you. The big question is: Is it beneficial in a large enough set of scenarios! \$\endgroup\$
    – Peter M
    May 4, 2015 at 18:44

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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.

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  • \$\begingroup\$ So how to DSLRs keep focus when shooting videos? There has to be more to it than that. \$\endgroup\$
    – Raphael
    May 5, 2015 at 0:19
  • \$\begingroup\$ With exception to the Canon EOS 70D, they use Contrast-Detect AF which can easily be detected by the back and forth movement of the lens when out-of-focus. The camera measures contrast and moves the lens to see if it increases. If it decreases, it starts moving the lens the other way. Sony SLT cameras were designed exactly to avoid that problem. Canon fitted the 70D with phase-detection at every pixel so it can continuously adjust focus while filming. For video, one should really focus manually because any misfocus, including locking focus on the wrong subject, will be visible in the film. \$\endgroup\$
    – Itai
    May 5, 2015 at 2:07

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