Demystifying Flash Duration

The Flash Duration for any flash is technically is the length of time that the flash tube emits light for a single burst. Unfortunately, the flash manufacturers don’t tell you how long the actual flash duration lasts. Instead most manufacturers confuse you in their literature as the t0.5 or the t0.1 flash duration specification. The main reason they don’t tell about the total flash duration is that the flash does not have a constant intensity over its entire duration. The flash intensity is brightest just after the flash is triggered and then generally falls to zero.

These flash duration specification values are part of the standards defined by the International Organization for Standardization (ISO). ISO Standard 2827:1998 which defines two values for quoting flash duration:

• The effective flash duration t0.5 is defined as being the period during which the flash intensity exceeds 50% of its maximum. 50% of the flash power is produced after t0.5 has been reached.
• The total flash duration t0.1 is defined as being the period during which the flash intensity exceeds 10% of its maximum.

The t0.5 specification is the period during which the flash intensity exceeds 50% of its maximum brightness. What this means is that 50 % of the light is emitted after the t0.5 threshold is met. Hence, as you can see from the picture above, the actual flash duration can be significantly longer than the t0.5 specification. What this means for the photographer is that there is still a lot of light being emitted after the t0.5 threshold and thus there is the possibility for motion blur in the image. For sports photographers, or any photographers trying to capture fast-moving subjects using flash, this is a major issue.

Because the meaning of the t0.5 specification is somewhat ambiguous, the ISO governing body set the t0.1 duration as the total flash duration. The t0.1 specification is the period during which the flash intensity exceeds 10% of its maximum brightness. As you can see in the Picture above, the t0.1 value is significantly more informative and much closer to the actual duration of the flash.

If you are confused with all this, here is the simple version. The t0.5 time measures the time it takes for your flash to go from off, to full power, then back down to 50% power (hence t0.5). The t0.1 does the same but measures all the way down to 10% power.

These two separate flash duration values allow us to compare the flash duration of various flashes accurately. For the outdoor photographer who shoot adventure sports or anything that moves quickly, selecting a flash with a very short flash duration is a must. Most manufacturers offer flash kits with extremely short flash durations that are marketed specifically to sports and action photographers. Some flash manufacturers even have flash cut-off technology built into their power packs, which reduces the flash duration significantly.

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 (with no ambient), then the speed at which flash turn on and off is very important.

Imagine a pitch-black room. You could take a 10-second exposure and your camera still picks up nothing. Now, take another 10-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. If not the subject will be frozen.

Bringing it back to our t0.5 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 t0.5 time) it’s still putting out 250 W/S. Meaning there is still light there which might expose the subject. On the other hand, the t0.1 of a 500 W/S flash will only be 50 W/S which might not be enough to expose the movement.

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 t0.1 and not t0.5 time, and the specific speed you need will depend on the action you’re photographing. The obvious example is of course to freeze motion; a person jumping, a dancer, liquids, powder and paint being thrown, anything moving quickly. The product photographs, as well as all those photos which are posted here like the waterdrop capture, do need fast flash duration to avoid blur.

The next time you’re considering a purchase make sure you know the t0.1 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.

Since most manufacturers give t0.5 values it is possible to approximately find t0.1 value. Typically, the rule is to divide by three. If the t0.5 is 1/3000 then the t0.1 is 1/1000. However, this varies depending on the 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 very expensive heads the numbers are close. Personally, if I can’t find the figures I err on the side of caution and divide by three.

Many photographers have tested flashes using the Sekonic Digital Light Meter and posted their results. You can check these two valuable sites.

https://gock.net/blog/2012/01/flash-durations-small-strobes/ is little dated.

This one lists most of the current line of flashes available in the market.
http://www.thebroketographers.com/blog/2017/4/13/flash-duration-analysis-with-the-sekonic-l858d-u

Many photographers shoot exclusively with available light because they’re intimidated by what they fear are the complexities of using an external source of light. While initially seems complicated for the uninitiated, shooting with simple speed lights or small studio strobes is quite simple.

I will be conducting the workshop which intends to demystify the secret behind flash photography. Demystifying Flash Photography Workshop in Mangalore (12th August) & Bengaluru (25th August). If you are interested in joining any of these workshops contact me at 9880744258.