Flash GN → Abertura

Guide Number: flash to aperture

The Guide Number (GN) is a constant that tells you a flash's effective range at ISO 100, quoted in meters or feet. The math behind it is N = GN / distance, with N being the aperture (f-number) you need for a correct exposure. So a flash rated GN 40 fired at 5 m wants f/8. Most speedlights sit somewhere in the GN 12–60 m range, while studio strobes push past that. Move twice as far away and you give up 2 stops, courtesy of the inverse-square law. TTL (Through The Lens) handles exposure for you through a metering pre-flash, but the GN is still the number you reach for when working manually. Bear in mind that ISO and the zoom-head setting both rescale the effective GN.

Applications: studio, event and macro

You will see it in the studio with off-camera flash and softboxes. It shows up at weddings and events for bounce flash and fill, in macro with ring or twin flashes fired up close, in manual portrait work where the lighting is controlled, and in strobist setups where the photographer sets power from a GN calculation instead of leaning on TTL.

FAQ

How does ISO change GN? Every stop of ISO bumps the effective GN by a factor of √2. That means ISO 400 doubles the ISO-100 GN, and ISO 800 takes it up about 2.83 times.

What if I use a softbox or umbrella? Any modifier eats 1–3 stops of output. Don't trust the bare-flash GN here; reach for a flash meter or fire off a few test shots.

GN in meters or feet? Both conventions are out there, so always check which one you're reading. GN 40 m is about GN 131 ft. Mix the units up and the aperture you calculate will be way off.