Understanding High Speed Sync — The Photo Video Guy

It’s fair to say that most all photographers who use a flash at any time know that any camera with a focal plane shutter has a limitation on the shortest duration shutter speed.

The reason for this is simple. Focal plane shutters, such as those found in all DSLR and Mirrorless cameras with interchangeable lenses use two parts in the shutter called curtains. When the shutter release is engaged, the first curtain opens and when the shutter time expires the second curtain closes. As we decrease the open time of the shutter, which is referred to as increasing the shutter speed, the second curtain starts closing before the first curtain has completed opening. What we then have is a moving slit that exposes the sensor at these shorter duration shutter speeds. What we call the flash synchronization speed is the shortest duration that the camera can deliver where the first curtain has opened fully before the second curtain starts closing

Typical Flash Sync Speeds

When we were using film cameras, most focal plane shutters traversed horizontally and in general the flash synchronization shutter speed was 1/60th of a second. When the shutter traversal was changed to a vertical movement, we saw the flash synchronization shutter speed rise to 1/100th of a second and sometimes 1/125th of a second. The shorter traversal distance allowed for the shorter duration shutter speed.

In our digital and mirrorless interchangeable lens cameras today, we find the flash synchronization shutter speed to be somewhere between 1/200th of a second and 1/250th of a second. There are outliers that are slightly faster, and a few that are slower. We have reached the logical maximum for a mechanical shutter and for the most part this is sufficient overall.

Shutter Speed Duration and Flash Duration

Most flash makers today talk about power or output level control. What they are actually talking about is flash duration. Electronic flashes apply a charge to a gas in the flash tube, typically Xenon that generates a burst of light when the charge hits it. This burst is near instaneous, very different that the measureable rise time for old flash bulbs that needed to get the charge across magnesium filaments inside the sealed flash bulb.

In a modern hot shoe style flash, a full power pop may have a duration as long as 1/800th of a second. Different designs could deliver a flash with the same measureable output at full duration where that time is only 1/1900th of a second. Most users are unaware of these differences and in normal application, they are irrelevant.

Why is Flash Duration Not Normally a Consideration

Let’s use the example of a DSLR or Mirrorless camera whose documentation says that the flash synchronization shutter speed is 1/200th of a second. When we release a shutter set to this speed, the entire face of the sensor is exposed at the same time. This will be true for every slower shutter speed from the flash synchronization shutter speed on down.

In general, unless otherwise configured on cameras that support an alternative, as soon as the first curtain is fully open the flash is triggered. If we use the example of the flash with the longest duration of 1/800th of a second, we can see that the flash duration is much shorter than the camera shutter speed, and can understand that once the flash finishes, it is not providing any more light to the sensor until the second curtain closes.

As we “turn down the power” on a flash, we are not actually changing the power at all, instead we are reducing the flash duration which delivers less light because of the shorter duration. Makers refer to this as power because it is easier to understand than duration in general, although high speed photography definitely cares about duration. In general a modern hot shoe flash will have durations as short as 1/8000th of a second when the “power is turned all the way down”. This depends on the flash design as some may support shorter durations.

Why Short Flash Duration Has No Impact on Flash Synchronization Shutter Speed

We now understand that even at “full power” or maximum flash duration the flash has finished its firing cycle long before the second curtain starts moving. So what happens if we shorten the shutter open duration, aka increase the shutter speed?

When we increase the shutter speed beyond the flash synchronization shutter speed, we know that the second curtain starts closing before the first curtain reaches it’s fully open state. Instead of having the full sensor exposed at the same time, we have a moving slit, where the higher the shutter speed is set, the smaller the slit. The result is a photograph with a portion lit correctly and a portion dark, because the second curtain was already blocking the sensor when the flash was triggered. This proves to use that flash duration and shutter open time are not directly linked.

When Do I Need a Higher Flash Synchronization Shutter Speed?

Let’s consider the scenario where you are photographing a couple outdoors on a sunny day and want to use flash to fill in the shadows created by the harsh sun. If we use the Sunny Sixteen Rule for an ISO setting of ISO 100, the rule tells us that at ISO 100 and an aperture of f/16 the shutter speed would be 1/100th of a second. That shutter speed is less than the flash synchronization speed so all would be great.

Except that f/16 delivers plenty of depth of field and you actually want to create separation between the couple and the background by reducing the depth of field. You choose to do this in camera because masking the couple in post-processing, inverting the mask and applying a Gaussian Blur filter is going to take time and will never look the same as a natural fall off of depth of field. You determine that to achieve the depth of field that you want to produce sharpness in the couple and a nice background blur would require an aperture setting of f/2.8 You quickly determine that this is a difference of 5 stops of exposure. Your camera will not go to a lower ISO, so the only option is to modify the shutter speed. A 5 stop increase in shutter speed to get the necessary shorter exposure duration would require a shutter speed of 1/3200th of a second.

While your camera can deliver that shutter speed, flash is not going to work because of that very high shutter speed.

Included below is a scenario where the scene requires a much higher shutter speed than the flash synchronization shutter speed. Our demand is a very large aperture and we want to fill with flash. The first image is too hot because I used the flash synchronization shutter speed. The second image uses a higher shutter speed and a 2 stop smaller aperture but the High Speed Sync function was not activated and consequently the shutter had already started to close when the flash fired. The third shot mixes ambient and flash as noted and even at 1/2000th of a second, the HSS function ensured that there was no part of the image darkened because of the moving shutter curtain.

*Sample images showing the mixing of Ambient Light and flash with high speed sync*

Enter High Speed Synchronization aka HSS

HSS, has to be available as a function on your camera and your flash. If the function is not present on both, you cannot use it. Let’s say that the function is available on both. You set the camera and the flash to their High Speed Sync modes.

Now as that small slit moves across the sensor to make the exposure at 1/3200th of a second the flash fires multiple times during the shutter traversal. You don’t have to calculate how many pops are going to be needed, that is done by the computer inside the camera.

However, we know that all electronic flashes need some time to recharge between flashes, and a full power recharge will take at least one second on a really powerful flash, and that is much longer than your total shutter open duration. The HSS function limits the output duration of each of the pops during the exposure to deliver proper exposure through the curtain slit for the duration of the exposure.

This means that each pop has considerably less output in HSS mode to keep the recycle time inside the shutter speed set. This can mean that you may need to move your flash closer to the subject so that it does not have to work so hard and can then generate enough pops to ensure that there are no dark bands on the exposure because the flash did not go off.

That it can do this is absolutely wonderful. You can now use flash outdoors with shallow depth of field in bright scenes! What you will want to know is what the limitations are for HSS on your camera and your flash. For example, your camera may have a shortest shutter open time of 1/8000th of a second, but the HSS function may only work as high as 1/2000th of a second, so checking your devices’ documentation is critical.

In the next two sample images we see the same instrument photographed using flash as the dominant source. The first image is shot at 1/200th of a second at f/4.0 and ISO 100 using the flash in TTL mode. The second image is shot at 1/500th of a second at f/4.0 and ISO 100 using the flash in TTL mode with High Speed Sync enabled.

1/500, f/4.0 ISO 100 HIGH SPEED SYNC TTL FLASH

The difference is that I moved a bit between shots and the second shot used 1/500th of a second as the shutter speed and had High Speed Sync enabled so the flash popped multiple times.

In Summary

High speed sync, if available on your flash and camera, or in many cases even if only on the flash, provides you the opportunity to use higher shutter speeds than the flash synchronization shutter speed when you need it.

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