Flash ISO GN Distance Aperture

Flash Guide Number, ISO & Distance

A flash’s Guide Number (GN) is the manufacturer rating that links aperture, distance and ISO together. Manufacturers nearly always quote it at ISO 100, which is why the working formula is distance = GN / N, with N standing for the f‑number. Raise the sensor sensitivity and the effective guide number grows as GN_ISO = GN_100 · √(ISO/100). Go from ISO 100 to ISO 400 and your reach doubles, since √4 = 2.

Lock the flash into manual mode and you skip the TTL pre‑flashes, getting output that repeats exactly shot after shot. That matters for portraits, product work and chained slaves alike. Pair the GN math with the inverse‑square law, where intensity drops with the square of distance, and you can predict how light falls off across a scene full of subjects and build the lighting ratios you actually want.

Applications

An event shooter dials in bounce‑flash power for a ballroom with it. A wedding photographer leans on it to balance ambient against fill during a reception. Product people work out how far the softbox needs to sit to land on f/11 or f/16. And strobists use it when chaining optical slaves, where every unit has to reach its subject at a distance they already know.

FAQ

Why does my GN seem lower than the spec sheet? Those marketing numbers assume the zoom head is racked all the way to tele (105 mm or 200 mm), with nothing in front of it. Pull back to a wide setting, clip on a diffusion dome or bounce the light, and the working GN drops.

Does HSS (high‑speed sync) change the GN? It does. HSS fires the flash in rapid pulses across the shutter travel, and that costs you somewhere around 2–3 stops of effective output, so the working GN shrinks to match.

Can I use this with studio strobes? Studio packs are normally rated in watt‑seconds (Ws) rather than GN. Take a flash meter or fire a calibration shot at a known distance to work out an effective GN for your modifier, and from there the same ISO and aperture scaling applies.