Glossary

Guide Number

Also called: GN

A flash's power rating, expressed as a distance times an f-stop.

If a flash has Guide Number 32 in meters, you can shoot at f/8 from 4 meters away (32 divided by 4), or f/4 from 8 meters away. Guide Number rises with flash power and falls inversely with distance. The app's flash calculator turns this math into a single answer once you tell it your flash GN, your distance, and your ISO.

Where this shows up in the app

6.2

How the Flash Guide Number works

f-number from a flash unit's Guide Number, distance, and ISO.

Tools tab > Flash Guide Number