Product Photography Lighting Calculator

This calculator applies the classic flash guide number relationship — GN = distance × f-number — to work out exactly how hard your key and fill lights need to work for a product shoot. It first computes the guide number required to correctly expose your subject at the distance and aperture you specify, then compares that against your flash's effective guide number, which is your flash's rated maximum GN scaled up by the square root of ISO/100 (guide numbers scale with the square root of ISO because exposure follows an inverse-square light relationship). Any light modifier — softbox, umbrella, diffuser — costs stops of output, and the calculator converts your specified modifier loss in stops into a guide-number multiplier of 2^(stops/2) — the square root of the linear 2^stops power-loss factor, since guide number itself scales with the square root of power — and folds that into the required guide number before comparing, since a diffused key light needs more raw power to deliver the same light at the subject.

The ratio of required-to-effective guide number, squared, gives the flash power percentage needed — squared because guide number scales with the square root of power, so doubling required output takes 4× the power setting. Fill light power is simply the key light's power divided by your chosen lighting ratio (key:fill), and that ratio is also expressed in stops via log base 2, since a 2:1 ratio is a one-stop difference and 4:1 is two stops — useful shorthand when dialing in mood versus flat, even lighting. Total watt-second draw is a rough estimate built from guide number squared, intended for gauging whether your kit has enough headroom, not as a precise power-meter reading.