Flash Guide Number Calculator

The guide number packages the inverse square law into a single figure. Illuminance from a point source falls as one over distance squared, while the light passed by a lens rises as one over the f-number squared, so for a fixed film or sensor sensitivity the product of distance and f-number is constant for correct exposure. That constant is the guide number: GN equals distance times aperture, quoted at ISO 100 and normally at a stated zoom head position. Rearranged, the aperture is GN divided by distance and the maximum working distance is GN divided by aperture, so a GN 58 (metres) unit covers 10.4 metres at f/5.6 or 5.3 metres at f/11. Sensitivity and power enter under a square root because both scale light linearly while GN scales with its square root: effective GN equals GN times the square root of ISO/100 times the square root of the fractional power. Going from ISO 100 to ISO 400 doubles reach; dropping from full to quarter power halves it. Typical ratings run about GN 20 to 28 for a small on-camera unit, 36 to 44 for a mid-range speedlight, and 56 to 60 for a professional unit, all in metres at ISO 100; a figure quoted in feet is 3.281 times larger and describes identical output, which is the most common source of confusion when comparing brands. Several factors qualify the result. Zooming the head from 24 mm to 105 mm concentrates the beam and can nearly double the quoted GN. Umbrellas, softboxes and ceiling bounce cost about one to two stops, and for bounce the relevant distance is the whole flash-to-surface-to-subject path rather than the direct one.

Photographic flash began with combustion. A magnesium flash powder mixture published by Adolf Miethe and Johannes Gaedicke in 1887 gave the first practical artificial burst for still photography, though it was smoky and hazardous. Paul Vierkoetter patented an enclosed flashbulb in the 1920s and General Electric brought bulbs to market around 1930, sealing the reaction inside glass. Harold Edgerton at the Massachusetts Institute of Technology developed the electronic xenon strobe in the early 1930s, producing repeatable short-duration flashes whose output could be specified numerically instead of estimated from a scoop of powder. Repeatability is what made the guide number possible. With no through-the-lens metering, photographers needed a way to convert distance into an aperture on the spot, so manufacturers printed guide number tables on flashbulb cartons, on the backs of flash units and on film datasheets, indexed by film speed and often by shutter speed for focal-plane synchronisation. Calculator dials on the flash body let the user set the film speed, read off the distance scale and transfer the aperture to the lens. Automation gradually displaced the arithmetic: thyristor auto flashes of the 1970s used a sensor on the flash to quench the tube, and Olympus introduced off-the-film metering in the OM-2 in 1975, measuring the flash exposure at the film plane during the exposure itself. Guide numbers survived because they remain the only comparable specification of raw output and the standard tool for manual and off-camera lighting.