Flash Duration
How long a flash's burst of light actually lasts, which sets its ability to freeze motion independently of shutter speed.
Formula / rule
t.1 ≈ 3 × t.5
Measured in
seconds
In depth
When flash dominates the exposure, it is the flash duration rather than the shutter that determines motion blur. Manufacturers quote two figures: t.5, measured between the half-power points, and the more honest t.1, measured at a tenth power. Most units get faster as power is reduced, though some do the opposite by truncating the tail.
Examples in practice
A flash with a t.1 duration of 1/8000 s freezes a splash of milk sharply even at a shutter speed of 1/125 s.