Camera flash pulses are typically very fast. It is fast enough to freeze motion even if you use a very slow shutter speed as long as the light from flash dominates the exposure.
Most camera flashes have xenon flashtubes. It lights up when the charge stored in a capacitor is discharged through xenon gas. The xenon gas is not very conductive in its normal state but its resistance can be greatly reduced when the xenon gas molecules are ionized, ignited by a high voltage pulse. The charges stored in the capacitor start to flow through the tube, giving out a very bright light. As the charge stored in capacitor discharges, the light intensity decreases.
The light intensity during a flash pulse isn’t constant. The light intensity emitted from the flash tube rises to a maximum quickly then fall off. If the charges stored in the capacitor are allowed to completely discharge, the light intensity will show a graduate reduction tail. Since we don’t want full power flash for each shot but rather just the right amount of light for proper exposure, the flash unit cuts off the discharge circuit at different times to control the amount of light emitted. Lower flash output power means shorter flash duration.
If you look at typical discharge curves (for example Sigma EF-500 curves or Canon 430EZ), 1/1 output power shows a long tail, 1/2 and other partial discharges show a very steep light fall-off traces. How exactly is the flash duration defined? According to this article, the flash duration is typically defined as the time during which the flash intensity exceeds 50% of its peak value.
For Nikon SB-800 autofocus speedlight flash, the following are the flash duration at different manual (M) output levels.
- M 1/1 output: 1/1050s
- M 1/2 output: 1/1100s
- M 1/4 output: 1/2700s
- M 1/8 output: 1/5900s
- M 1/16 output: 1/10900s
- M 1/32 output: 1/17800s
- M 1/64 output: 1/32300s
- M 1/128 output: 1/41600s
Keywords: Flash Duration, Flash photography