camera basics - What is flash duration?

When speaking in terms of flash specifications, what is flash duration and how does it impact exposure?

Answer

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.

1. 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