In a flash!

Your Speedlight’s flash duration changes based on its output. For example, at full power our SB-900 has a flash duration of about 1/880 sec. At its lowest setting – 1/128th power – the duration is an almost instantaneous 1/38,500 sec. The lower the power, the shorter the duration. For capturing splitsecond action you’ll need a dark room. This way the flash duration effectively becomes the length of the exposure, as the ambient light is negligible, and shouldn’t affect the exposure.