Flash duration is simply the duration time that the flash emits light when fired.  Essentially, this unit of measurement tells you how quickly the light goes from full power to off, with quicker times being more desirable than slower times.  For flash manufacturers, a quick flash duration has become a marketing point of emphasis.  But does it even matter?   This article will cut through the marketing jargon, 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 measured in T.5 and T.1 times.

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.

L'oreal age perfect serum surrounded by splashes. Used to demonstrate the freezing of liquid using flash duration
Photo by Square Mountain Photography

Why the T.1 Time is More Important Than the T.5 Time

The T.5 time is the amount of time it takes for a 50% of the flash power to dissipate. That time is actually the time that most manufacturers use when they tell you what the flash duration is.

There’s a problem with that. If only 50% of the power has dissipated, what about the other 50%? The other 50% is still going to affect your image. If the flash duration is slow, that 50% is going to actually affect something. What’s more adequate, what’s a better measurement of the ability of a flash to freeze or to know what the flash duration is the T.1. The T.1 time is the time for 90% of the light to dissipate and that remaining 10% that trails off is really not going to affect much.

That’s what really matters.

Power Settings and the T1 Time

There’s another interesting thing. The T.1 time is the best measurement of the flash’s ability to freeze a subject, but the higher the power that you use,  the slower the flash duration, so the slower that T.1 time.  The lower the power setting on your flash, the faster the T1 time, i.e. the faster the flash duration.

A pocket strobe is generally going to be faster than a studio strobe, generally. That’s why oftentimes when people are working with liquids, when people are working with close up macro dropping things into things like water, they’re using pocket strobes because they’re convenient. They’re easy to manipulate and so forth and their flash durations are very quick. To freeze water, you need a very, very quick flash duration, 1/5000th of a second, 1/10000 of a second to freeze water. Sometimes studio strobes can’t quite get there.

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, 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.

bottle of Dove body wash surrounded by wings. Demonstrating photographing slower moving objects.
Photo by Square Mountain Photography

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 Flash Training System.

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

Where to Find Accurate Flash Duration Times

If you want to get T1 times for a specific flash, you can look online. A great resource is actually from Andy Gock. Andy has done some fantastic testing on flash durations.

Oftentimes you can find the T1 time from the manufacturer themselves, but sometimes it’s not available. Just look online, you can find the actual flash duration. I would highly recommend doing that before purchasing new flashes because you might end up with flashes that might not have a fast enough flash duration for whatever use you might have. For most people, this generally is not an issue, but if you’re wanting to use your flash to freeze motion, this becomes a problem.

Conclusion | Next Steps

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.  If you’re looking for more information on Flash Duration and other topics, I would suggest watching the video embedded below: