Guide number (GN)
Richtgetal (GN) · Light
A number indicating the maximum power of a flash unit, expressed in metres at ISO 100.
The guide number tells you how far the light reaches: GN = distance × f-number. A flash with GN 60 correctly exposes a subject at 6 metres at f/10. The higher the guide number, the more powerful the flash. External speedlights have higher guide numbers than built-in flashes.
Tools that help with this: