Find a flash's effective guide number at a given ISO and distance.
The guide number is the aperture times the distance, and higher ISO effectively raises it because more light is visible. Guide numbers let you set a correct flash exposure or judge how far a compact flash can reach at a given aperture.
Flash Guide Number
GN = aperture x distance; scale by sqrt(ISO/100)
GN = aperture x distance; scale by sqrt(ISO/100) The guide number is the aperture times the distance, and higher ISO effectively raises it because more light is visible.
Guide numbers let you set a correct flash exposure or judge how far a compact flash can reach at a given aperture.
This calculator takes 3 inputs: Aperture (f-number), Flash-to-subject distance, ISO. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.