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Cutting Through The Crap | An Explanation Of Flash Duration

By Max Bridge on September 29th 2016

I wrote an article recently which explained why I paid well over the odds to purchase Paul C. Buff’s Einsteins and have them shipped to the UK. At roughly the same time, Profoto announced the D2 and Pro-10 which had extremely fast flash durations. Then just days later Hensel announces the Cito which boasts an even faster flash duration. During this time, especially with the publication of my article, I noticed an obvious amount of confusion amongst people concerning flash durations. This article will cut through the marketing crap, explain what flash durations are, when a fast flash duration is needed, and how some of you have been slightly mislead.

Misleading Flash Durations, How And Why?

‘Misleading’ may be too strong of a word, but you can decide for yourself in a minute. In very basic terms, the flash duration is a measurement of how long it takes a flash to go from full power to off; it’s given in T.5 and T.1 times. I’ll explain the difference between them in a second. What you need to understand for now, is that the T.5 time (the time that most manufacturers give) is pretty much a useless measurement for anyone interested in freezing motion. However, in marketing terms it’s the bigger number and therefore sounds better.

Most manufacturers do not give the T.1 times of their flashes. Something which I find immeasurably frustrating. You can sometimes find the T.1 time by doing a little online digging but often you can’t or it’s a true hassle. What’s worse, is that people purchase lights based on the T.5 figures and end up with equipment which is not suited to their needs.

slide from lighting 10 explaining flash duration

What Is The T.1 And T.5 Flash Duration?

So what’s the big deal? What’s the difference between those two numbers? It’s actually really simple. The T.5 time measures the time it takes for your flash to go from off, to full power, then back down to 50% power (hence T.5). The T.1 does the same but measures all the way down to 10% power. This is often represented in graph form which you can see in the photo above taken from Lighting 101.

If you want to freeze motion with ambient light only, you use your shutter speed. That’s probably something you’re familiar with. If you’re using flashes, and they’re your main source of light (no ambient), then the speed at which they turn on and off is very important.

Imagine a pitch black room. You could take a 30 second exposure and your camera still picks up nothing. Now, take another 30 second exposure but fire a flash at some point. The time it takes for that flash to turn on and off will determine the time it takes for your photo to be exposed. If the flash takes a long time, then a moving subject will be blurred and vice versa.

Bringing it back to our T.5 and T.1 times, if you’re firing a 500 W/S flash in our pitch black room, when it’s at 50% power (the T.5 time) it’s still putting out 250 W/S. That’s gonna have a BIG effect on our exposure, isn’t it? On the other hand, the T.1 of a 500 W/S flash will only be 50 W/S.

If this has all become a little confusing for you, just remember this. If you want to freeze motion, your flashes need a fast flash duration, the measurement to take note of is the T.1 time, and the specific speed you need will depend on the action you’re photographing. That’s it. The last part (the speed you need) is hard to know, but you can find it.

OK, Why Do I care? What Can I do With This Knowledge?

The obvious example is of course to freeze motion; a person jumping, a dancer, liquids, powder and paint being thrown, anything moving quickly. In the photo you see above, I used some stock splashes which were taken from the Photigy splash pack (find it here), but those splashes are captured by product photographer Alex Koloskov; he uses some of the best equipment to ensure the splashes are completely frozen. Another example from my own work is this “Dove With Wings” image. That was accomplished by composting evaporated milk, which had been injected into a fish tank, find a full explanation of that method here. In that example, it’s possible to use flashes with a slower flash duration as the subject (evaporated milk) is not moving as fast.

It’s not only useful to know this stuff for specialist types of photography like my product work. By understanding flash durations, exposure times and the effect of camera movement, Pye and the Lin & Jirsa team create unique shots which amaze their clients. In a recent article Pye explains how to take dance floor images like the one below, check it out here.


two women on dance floor at wedding with light trails surrounding them. Example of using flash duration creatively

A variation on the same technique, allows them to pull off stunning shots like this one. If you’d like to learn more about these techniques, the SLR Lounge team have crafted some wonderful tutorials which you can find in the SLR Lounge store, click here. I’d encourage you to look over Lighting 101 and Lighting 201.

a couple surrounded by light trails. Used to demonstrate the creative effects of flash duration.


The next time you’re considering a purchase make sure you know the T.1 times of your flashes. For many photographers, this won’t matter too much. If your subjects don’t move then it doesn’t matter at all. However, the day may come where you need those fast flash durations but don’t have them. If it were me, and I had the option, I’d always choose a flash which also had good flash durations just in case.


Can I work out the T.1 flash duration from the T.5? Yes and no. Typically the rule is to divide by three. If the T.5 is 1/3000 then the T.1 is 1/1000. However, this varies depending on flash brand and technology used. For cheap brands, I’ve heard people say you need to divide by four, whereas I’ve also heard people say that with some IGBT heads the numbers are pretty close. Personally, if I can’t find the figures I err on the side of caution and divide by three.

As to whether this is misleading or not, that’s for you to decide.

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Max began his career within the film industry. He’s worked on everything from a banned horror film to multi-million-pound commercials crewed by top industry professionals. After suffering a back injury, Max left the film industry and is now using his knowledge to pursue a career within photography.

Website: SquareMountain 
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Q&A Discussions

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  1. Dylan Richardson

    Good article, but I find it odd that manufacturers are still using watts as an indicator of power or luminescence since this is actually a measure energy used.

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  2. Kim Bentsen

    Good News. The new Sekonic L-858D announced at Photokina allows you to measure flash duration. Now flash units can be compared between brands and models, but also at different power settings.

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  3. robert raymer

    Great article. I found that Paul C Buff (the company that makes Alien bees and Einsteins) also has an excellent section on their website with 10 or so short articles explaining the ins and outs of studio flash.

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