Find the aperture or reach your flash can manage from its guide number.
Guide number, distance and ISO combine to give the aperture needed for correct flash exposure, and the equation reverses to show reach. Flash exposure fails as distance grows, so the guide number equation predicts the working range before the shot is taken.
Flash Exposure
f-number = guide number / (distance x sqrt(ISO/100))
f-number = guide number / (distance x sqrt(ISO/100)) Guide number, distance and ISO combine to give the aperture needed for correct flash exposure, and the equation reverses to show reach.
Flash exposure fails as distance grows, so the guide number equation predicts the working range before the shot is taken.
This calculator takes 3 inputs: Guide number, Distance, ISO. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.