What does flash duration mean, and how does it affect exposure?

Asked 7/28/2010

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I often see flash duration listed in flash specifications. What exactly does flash duration mean, and how does it influence exposure and the ability to freeze motion?

lighting high-speed-photography flash-duration flash exposure

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

16y ago

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Simply, it's the duration the flash is actually on, emitting light.

This doesn't impact exposure per se; as long as the same amount of light is emitted, you'll get the same exposure. But it can affect the result: as the flash duration gets shorter, it has a better ability to freeze motion.

For most photography this won't matter very much. 1/1000s is a typical duration for decent flashguns at full power1, which is more than sufficient to freeze normal motion in a photograph. Other aspects will have much more bearing on the results; flash power, shutter speed, and so forth.

The one example I can imagine where shorter flash durations would be particularly desirable is for high-speed photography.

Studio strobes are sometimes rated faster, but thanks to their different characteristics, may be slower in practice. More detail here: http://www.scantips.com/speed.html

Originally by user496. Source · Licensed CC BY-SA 4.0

user496

16y ago

AI Answer

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Flash duration is the length of time the flash actually emits light.

In terms of exposure, it matters because the flash is only adding light while it is on. But if two flashes deliver the same total amount of light, they can produce similar exposure even if one does so in a shorter burst.

Where flash duration really matters is motion freezing. A shorter flash burst can stop faster movement, because the subject is only illuminated for a very brief time. This is especially important in high-speed photography, such as water drops or splashes.

In some setups, the shutter speed is not what freezes the action at all. For example, in a very dark room you can leave the shutter open relatively long and rely on the short flash burst to capture a sharp instant.

So, in practice:

flash duration = how long the flash is on
total flash output affects exposure
shorter flash duration improves motion stopping
it is most critical for high-speed subjects, less so for typical everyday flash use

What does flash duration mean, and how does it affect exposure?

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