Design & colour
Flash guide number calculator
Relate ISO, direct-flash distance and aperture using a supplied guide number.
A clear answer. Here’s how.
ISO-adjusted guide number = GN100×sqrt(ISO/100). Suggested f-number for the supplied distance is adjusted GN/distance; distance at the supplied f-number is adjusted GN/f-number. Uses the same direct-flash beam/power conditions as the guide number you enter; bounce, diffusers and high-speed-sync losses are excluded.
See a worked example
Worked example
A fixed demonstration, separate from the values you enter above.
- Guide number at ISO 100 (metres)
- 34
- ISO setting
- 100
- Flash-to-subject distance (m)
- 4.25
- Aperture for distance estimate
- 8
Example result
- ISO-adjusted guide number (m)
- 34
- F-number for supplied distance
- 8
- Distance at supplied aperture (m)
- 4.25