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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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3 Answers 3

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You make the assumption that light drops significantly between two flashes. This isn't true:

enter image description here

(full story at https://neilvn.com/tangents/high-speed-flash-sync/)

Since doing this properly is likely dependent on the light dropout characteristic of the flash bulb, it can be done as an open-loop.

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  • \$\begingroup\$ The drawing seems a bit misleading, in order for there to be successive ON states there must be an OFF state in-between them. \$\endgroup\$
    – Steven Kersting
    Sep 11, 2019 at 21:43
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    \$\begingroup\$ The drawing is really about the light output. See the drawing above for low speed. \$\endgroup\$
    – xenoid
    Sep 11, 2019 at 22:30
  • \$\begingroup\$ Oh, that certainly updates my understanding of how things work! \$\endgroup\$
    – juhist
    Sep 12, 2019 at 5:58
  • \$\begingroup\$ @StevenKersting The power is switched off and on very rapidly, but the energy still being dissipated by the gas in the bulb never dims completely. \$\endgroup\$
    – Michael C
    Sep 12, 2019 at 15:30
  • \$\begingroup\$ @ Michael C I am aware, the on/off cycle is much faster than shown and more similar to the waveform depicted... I just felt the statement that the light not dropping between two flashes, and the diagram omitting the off state, both potentially a bit confusing for the OP. \$\endgroup\$
    – Steven Kersting
    Sep 14, 2019 at 20:35
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Answering the question(s) directly,

Is HSS open or closed loop?

It is open loop. The flash firing and synchronization is not performed by measuring the flash's light incident on the sensor.

HSS is camera-manufacturer specific, requiring the flash to understand the camera's proprietary signaling and timing commands.

Does it work in complete darkness?

Absolutely.

However, even if it were closed loop via the imaging sensor as the feedback element, that would not, a priori, preclude any HSS system from working. By analogy, closed loop stepper motor control works with widely varying loads at motor startup. If the feedback controller has fast enough slew control, a nominally safe starting speed/torque can be used until the feedback loop can establish what the (commanded – measured) difference is. Similarly, a camera could measure and establish the flash's delay and intensity variation at the initial stages of shutter transition, if the camera were designed to do so. But it's moot, since the operation is open-loop.

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Many artificial light sources are actually pulsed and not continuous, such as LED's and fluorescent bulbs. They simply pulse at a rate fast enough so that the perception of them is that they are continuous. That is all that HSS is doing.

In order to do that it has to provide all of the light pulses from a single capacitor charge, so the max power is reduced to ~ 1/4 (typically). In most implementations there is no consideration for the shutter/transit speed because it is unnecessary.

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  • \$\begingroup\$ You're not wrong, but I feel your answer oversimplifies the issue. I believe there is consideration for the shutter/transit speed, because HSS capability is specific to camera manufacturer's protocol. HSS is not just a capability of the flash, nor just the camera — there is precise timing communication from the body to the flash to achieve the approximate flat-top luminous intensity from the flash bulb and resultant even exposure across the sensor. \$\endgroup\$
    – scottbb ♦
    Sep 11, 2019 at 22:56
  • \$\begingroup\$ The only thing precise about the timing is that HSS requires the "start signal" to be advanced and occur before the shutter opens. The camera does that automatically when it switches into HSS. There are numerous third party flashes where it is simply a flash capability... a button you push to put the flash into HSS (pulsed flash) mode. To a very large extent it is no different from the stroboscopic flash mode some flashes offer where it will flash multiple times during an exposure. The only difference is that stroboscopic mode usually offers you a choice of flash pulse timing. \$\endgroup\$
    – Steven Kersting
    Sep 11, 2019 at 23:21
  • \$\begingroup\$ (I apologize because I reposted an edited comment because it originally sounded like I was implying that only 1st party flash+camera combos are generally possible. I didn't mean to imply that). You're right that there are numerous 3rd party flashes where it is just a feature of the flash, but that timing is very much specific to the brand of camera it is attached to. \$\endgroup\$
    – scottbb ♦
    Sep 11, 2019 at 23:30
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    \$\begingroup\$ Once a flash is operating in HSS mode it is effectively a constant light source and is suitable for any available shutter speed/transit time... the only caveat being the start signal timing. But there's no point to loosing the flash power at SS's slower than shutter transit time (x-sync). The start signal timing is entirely controlled by the camera... Edit: tail sync/hyper sync timing can be controlled by the transmitter or receiver to set an additional delay which is a bit different. \$\endgroup\$
    – Steven Kersting
    Sep 11, 2019 at 23:33

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