Flash duration explained

Est. reading time: 4 - 5 minutes

When it comes to using studio flash, one of the things that often causes confusion is flash duration.

I often get asked what flash duration is, how to measure flash duration or which flash reading actually matters, or new photographers often don't understand why shutter speed isn’t important for freezing motion with flash.

I explain all of this in my ‘Understanding flash duration’ class, but I’ve also put together a quick summary here to help you understand this important concept.

What is flash duration in photography

Flash duration refers to the length of time of the burst of light that comes out of a studio flash. Quite simply, it is the duration for which a studio flash or speedlite emits light for a single burst.

This can vary across brands and also types of studio lights, but is always shown as one of two measurements: t0.5 or t0.1.

The t0.5 value measures the time it takes for the flash to reach full power and then reduce to 50% power. The t0.1 value, on the other hand, measures the time it takes to reach full power and then drop to 10% power.

Although many manufacturers show the t0.5 reading, the t0.1 reading is actually the most important because it accounts for nearly all of the whole flash burst, rather than just the brightest part of the flash.

Flash duration chart
Flash duration diagram taken from ‘Understanding flash duration' class.

Because the t0.5 reading doesn’t account for the majority of the flash burst, there is a much greater chance of there being some sort of motion blur in the image, which is not what we want when photographing fast-moving subjects.

Unfortunately, though, not all manufacturers list the t0.1 reading, often because the t0.5 reading is shorter and therefore sounds more impressive. So if you’re looking to compare different flash systems, make sure you’re looking at the t0.1 reading and that you’re comparing apples with apples.

Karl explaining flash duration

Understanding flash duration

What flash duration means and its effects.

Karl explains what flash duration is and demonstrates how different flash durations are able to freeze movement.
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Adjusting the flash duration

Shooting with studio flash allows us to capture and perfectly freeze very fast-moving subjects, for example, splash photography.

When capturing high-speed motion, the flash duration required to freeze the subject depends on a number of factors, not just how quickly the subject is moving. For example, flying liquids will require faster flash durations than a model jumping, and photographing a close-up splash shot will require a faster flash duration than a wide view of a splash shot.

Changing the flash duration is is often a case of adjusting it on the light itself (although it isn't as easy on speedlites). To achieve faster flash durations it's often necessary to work at lower powers. Reaching the desired flash duration comes down to a mix of power and choice

For example, the broncolor Scoros I use have a range of 1/85 – 1/10’000s (this is the t0.1 measurement). This means that at higher power settings I may only be able to achieve a flash duration of 1/85, but at lower powers I can achieve a much faster duration of 1/10’000. Generally, the power needs to be below 5.4 to get to the fastest flash duration.

Flash duration for freezing action

If you’re photographing portraits or products, then a fast flash duration isn’t something you need to worry about because the subject isn’t moving. However, if, like me, you often photograph models in motion or flying liquids, where you need to perfectly capture and freeze a specific moment in time, then flash duration is incredibly important.

For high-speed motion, such as liquids, I generally recommend a flash duration of at least 1/3000th at t0.1, but when photographing models jumping, for example, slower durations such as 1/2800th will still be sufficient.

These are just rough guides; as I mentioned earlier it all depends on how much motion and how wide-angle or close-up your shot is (the magnification).

If you don’t have studio lights with a fast flash duration then speedlites, or strobe lights, also offer a very fast flash duration, but the problem with these is that they don’t have a lot of power compared to studio lights.

Having said that, I used speedlites for a lot of my earlier splash photography, including my paint explosion image and liquid fashion image. I was able to overcome the problem of insufficient power by grouping multiple speedlites together and sometimes combining them with studio flash.

Flash duration & shutter speed

So what role does the shutter speed play in freezing motion with flash? The simple answer to this is ‘none’. When using studio flash (or speedlites), it is the flash that freezes the motion.

This is a whole other concept, which I explain in much more detail in my ‘The relationship of shutter speeds and apertures to flash’ class. I highly recommend watching this class if you want to better understand this concept.

Karl explaining cameras

The relationship of shutter speed and aperture to flash

How it all works together.

Karl demonstrates how shutter speeds, apertures and studio flash work together to influence the final image.
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Once you understand the relationship between shutter speed and flash, you’ll be able to use creative techniques like combining a fast flash duration with a slow shutter speed to freeze some motion in the image while allowing other elements to be blurred. You can see an example of this in my ‘Seascape fashion photography’ class.

The difference between flash duration and sync speed

There’s often some confusion about sync speed and how this relates to flash duration. Sync speed refers to the fastest shutter speed you can use when shooting with flash, without the shutter obscuring part of the image.

Every camera has a set flash sync speed listed, for example 1/125th or even 1/200th, and this essentially tells us the fastest shutter speed at which the camera and flash can sync.

Camera sync speed
Camera sync speed
Camera sync speed
Camera sync speed

A series of images showing varying degrees of sync speed problems.
Images from 'The relationship of shutter speed and aperture to flash' class.

Flash duration is often very fast, which leads a lot of people to ask why we can’t get a flash duration of 1/2000th, for example, inside a shutter speed of 1/1000th?

This is a good question, and it all comes down to the mechanics of your camera’s shutter.

The problem isn’t that the flash duration isn’t fast enough, it’s that the mechanics of the shutter (specifically focal-plane shutters) change as the shutter speed gets faster, which means the flash is unable to expose the entire image in one exposure (I show how this works in my ‘The relationship of shutter speeds and apertures to flash’ class).

It’s also worth mentioning that while most cameras have a set sync speed, cameras that use leaf shutters, like my medium-format Hasselblad, are able to sync at any shutter speed.

These types of shutters are located in the actual lens, rather than the camera, and work similarly to an aperture opening and closing, which is why the shutter can react more quickly (and also why these lenses tend to be more expensive).

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To learn more about flash duration through practical examples, take a look at our product photography course and fashion photography course, where you'll find a selection of classes that show how to freeze moving subjects using flash. Below are just a couple of our most popular classes that you may enjoy.


  1. Would you be so kind to answer a few questions I have regarding flash duration and flash sync speed?

    So if Im working with a shutter speed that is below the sync speed of my camera I can still control the flash duration independently? Or does the flash duration is automatically adjusted upon selection of my shutter speed whether that is below or above HSS?

    Also, when selecting a very fast shutter speed of let’s say 1/4000s and enabling HSS on the strobe does the strobe emits a series of microbursts of light (not visibly discernible) throughout the “scan”?
    Does the flash duration refer to the duration of each burst of light that is being output while scanning the sensor plane?

    I truly appreciate your patience while I am trying to sort this out and have a good grasp of the technology behind HSS.

    Thank you.

    1. Hi Myles, your shutter speed doesn’t control the flash duration. The flash duration is only set on the flash. On very high speed flashes like Siros or speedlites you only get super fast flash durations on the lower powers of the unit. If using HSS modes on most flashes the flash duration is actually intentionally longer and brighter so that your shutter speed will be quicker and more powerful (but still time with) than the flash burst to arrive at the correct exposure. I’m not familiar with any flash systems that would use bursts of light for HSS through out the scan.

      1. Hello Karl,
        at 7:06 of “The Relationship of Shutter Speeds and Apertures to Flash” you state that when working at shutter speeds above the manuf. sync speed the flash will fire a “longer and more powerful” burst of light for entire “scan” of the medium (sensor or film). However in your “Understanding Flash Sync Speeds” on Youtube (https://www.youtube.com/watch?v=0j5AHzAGaFk) at 10:11 and 10:23 and 10:35 you state the when using high speed sync technology, several bursts of light at lower power are fired throughout the entire exposure of the medium. Since these are pulses of light (10:11) and to be so fast and sequentially they need to work at lower power. To me actually your explanation on your Youtube channel makes much more sense. However my question is, what happen if I employ longer flash durations along with fast shutter speeds above the sync speed (say 1/300 shutter speed and 1/80s flash duration)? Wouldn’t I potentially create a situation where there is actually more motion frozen in the ambient light exposure and motion blur resulting from the flash exposure? Is that even possible?

        Thank you.

        1. Hi Myles, I say that with certain flash systems that have HSS then the flash will fire a longer burst for use with faster shutter speeds that are beyond the normal sync speed. There have also been systems that pulse the light rather than a longer burst but again this only applies to HSS technology. If you tried to do this yourself without HSS compatible triggers and flash it will likely mistime and result in a black line across your shot.

  2. alimarri

    Hi karl

    I have a question about the energy that each glash burst consumes.

    For example, i have a 360 jules ( or 360 watt/s) speedlight and in full power 1/1 , the t0.1 is 1/220 seconds.

    Is it correct that the energy of one ssingle burst is 360/220 (= 1.63 watt )?

    I will be happy if you tell me a little about flash energy consumption .

    Thank you

    1. Hi, if you are using your 360J light at full power then you are using all 360J in one burst of light. If you are using it at 1-fstop less then you are using it at 180J of power in one burst of light. The speed of the burst of light is usually faster when there are less Joules used. For example on very fast flash duration model lights when you get down to about 100J then some studio lights such as Siros or Scoro can acheive 1/10,000th of a sec t0.1 speed of burst.

      1. alimarri

        Thank you for your answering

        Does it means if we assume that this lamp could be turned on for 1 second, it will produce about 79000 jules and according to the watt formula ( j=w.s) its Power will be 79000 watts?

  3. Hi Karl!
    I want to shoot a model with jumps. I will use a short pulse battery pack as my main focus light. It is guaranteed to freeze the object. Backlight will be with the usual candy bar. Stripboxes or standard reflectors with a honeycomb are used as modifiers for the backlight. How big do you think the influence of these sources will be? Will I be able to freeze the object with this combination? I am going to shoot in a rented studio so there is no way to do a test.

    1. Hi AlexD, to be honest it all depends on the duration speed of your backlights and how bright they will be in relation to your front key lighting. If they are quite significant in proportion to the overall shot and they are not fast flash duration then it is likely you will see some ‘ghosting’ in the image I’m afraid.

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