So you’ve been reading the forums and keep reading about flash durations and how it is so important, but you don’t fully grasp why. This short article is my attempt to clarify what flash durations are, how come certain flashes are better than others and what you should look for in a flash.
Why is flash duration important?
An action sports photographer shooting without flashes relies on a high shutter speed to freeze the action of an athlete. If you’ve been shooting sports, you’ll know that if you drop your shutter speed too low, the action becomes a little bit blurry, often called motion blur. When flashes are introduced to the equation, the photographer is limited by his cameras sync speed, often in the range of 1/200th - 1/500th of a second. To compensate for shooting with this slower shutter speed, the flashes must output more light than the ambient (or available light) and must do so in a very short burst.
How does it work?
The basic set up is to have your flash power be 2 stops more powerful than your ambient light, and to have the flash fire for a very short amount of time. You then set your camera at the exposure of the flashes, not the ambient. When the flash fires it illuminates the subject and burns them over the background. If the duration of the flash firing is short enough the subject is burned only for that short amount of time, and appears frozen in the final image.
You can picture it by shooting a 2 second exposure in a pitch black room, if you were to fire your flash once at a subject running through that room, you would have a frozen frame of that person running (provided your flash only fired for a quick instant).
This is where the flash duration becomes important, if your flash was firing for 1/200th of a second (similar to using a shutter of 1/200th of a second) on the fast moving athlete, the subject would be blurred. We know already that to freeze action we need to use a shutter speed of 1/1000th or faster, so our flash duration should be the same 1/1000th or faster. Because we are overpowering the ambient, the slower shutter does not directly affect the motion blur. Try experimenting towards sunset, you’ll notice that sometimes you can drop your shutter to as low as 1/10th of a second (or lower) and get a crisp image, provided your flash durations are fast, and you meter properly.
How do camera sync speeds play in?
The main advantage to a camera with a high sync speed, is to get that extra stop of light. For example consider 2 cameras, 1 with a sync speed of 1/250th, and the other with a sync speed of 1/500th. If the ambient light is ISO 100, F8, 1/250th camera 1 will need to get flash power of at least F11 (preferably F16) to overpower the ambient, this means doubling up flashes. Camera 2 at ISO 100, F8, 1/250th is the same as ISO 100, F5.6, 1/500th so we can shoot at this setting since it syncs up to 1/500th, which means we only need to get our flash power up to F11 for 2 stops over the ambient.
Speedlights and flash durations
You have to be careful when purchasing speedlights to ensure that they have a fast enough duration. Many speedlights offer a fast enough duration at full power, have a look at the flash bible to see what the durations are of many of the flashes. If we don't have it listed, a quick search of google with the flash name and “flash duration” will hopefully give you the results you’re looking for. Some relatively inexpensive and popular speedlights include the Vivitar 285’s, Nikon SB 24,26,28 and 80’s which have a duration around 1/1000th at full power.
Sunpak 555’s and 544’s have been popular among skateboard photographers for some years now, however their flash duration at full power is not quite fast enough to freeze action, but changing the power output to 1/2 power solves this problem. At half power, most speedlights double the speed of their flash duration, be careful with studio strobes though, many studio strobes actually have longer durations at less power, which is the opposite of what we want.
My strobes flash duration is measured in t0.5 what is this?
Strobe manufactures generally use two different methods for measuring flash durations. You will normally see t0.5 beside the flash durations listed on a strobes specs (such as alien bees website), the flash duration generally seems pretty fast too, however this is misleading! t0.5 measures the flash duration for the period where the flash output is above 50% of the maximum flash power. This is great, but that other 50% matters! So whenever you’re checking specs be sure to find the t0.1 flash duration (measures duration of output above 10% of the maximum output).
Should I worry about TTL/eTTL/iTTL?
It’s always nice to have a TTL flash in your bag, but for action photographers, using your flashes off camera you definitely do not want TTL, you want to be shooting in full manual mode so that you can control the flash power of each flash individually to light your scene and subject properly.
Scenario: 1/250th camera sync speed
You are shooting a photo and measure your ambient light to be ISO 100, F5.6 at 1/250th. You have 2 options to freeze the action, you can increase your shutter speed to about 1/1000th and change your aperture / ISO to compensate, or you can use flashes to fill in your subject, you opt to use flashes. You set up 2 flashes as fill, one to your left, and one to your right. You meter them to give you an output of ISO 100, F11 at 1/250th. This is 2 full stops above your ambient light. You now set your camera to be ISO100, F11, 1/250th and prepare to make the shot.
I’m new to shooting with flashes what set up should i get?
If you’re new to using off camera flash, and don’t fully grasp what I said above, pick up 2 Vivitar 285’s and some sync cables to trigger your flashes and play around with that until you start to understand what’s going on. They’re also some of the cheapest (but still great quality and build) speedlights that you can buy. Trying things out is the best way to get an understanding of what’s going on.
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