Flash Duration Calculator — Motion Freezing

Flash duration varies with power level and flash technology:

Speedlight (IGBT)

Duration shortens at lower power. Each stop of power reduction cuts the duration by roughly 0.6–0.7× (the default model uses duration ÷ 2^(stops × 0.8); exact behaviour varies by model). At 1/16 to 1/32 power most speedlights reach 1/5000s or faster.

Studio Strobe

Duration behaviour varies by design. Many IGBT monolights shorten at lower power, while older voltage-controlled strobes can actually lengthen the t.1 tail when power is reduced.

t.1 vs t.5

t.1 ≈ 3 × t.5 for a standard RC-decay flash tube — t.1 (above 10% of peak) is the duration that actually matters for freezing motion, since the long tail still adds blur.

Freezing motion

A subject moving at speed v blurs by v × duration. To keep blur under ~1mm, the burst must be shorter than 0.001 ÷ v seconds — the calculator reports the maximum subject speed for that 1mm threshold.