Is HSS open or closed loop? Does it work in complete darkness?

by juhist   Last Updated September 11, 2019 16:18 PM - source

Some cameras and flashes have support for high-speed-sync for using flash with such high shutter speeds that the shutter is never fully open at the same time in all locations across the sensor area. See animation here for how fast shutter speeds work: If a rolling shutter travels from top to bottom, why does this image seem to show skew in the other direction?

Is this high-speed sync a closed loop or an open loop process? By this I mean: how does the camera decide when to do the next small flash? It has to be very accurately determined, or else some areas (horizontal stripes) may be exposed twice and/or not exposed at all.

One way the high-speed-sync could be implemented is that if there's some light (not complete darkness), the camera uses continuous sensor readout to determine when the shutter curtain moves past the last horizontal line that was exposed. Then the next small flash is properly synced to the previous small flash, thus avoiding the problem of horizontal stripes exposed twice and/or not exposed at all.

Now, if the HSS is implemented by a closed loop continuous sensor readout, it wouldn't work in complete 100% darkness. Is this actually the case? Or can you use HSS if there is absolutely no light at all (like in a darkroom)?

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