Use guide numbers to calculate flash exposure. Solve for aperture, distance, or required guide number.
GN = Distance × Aperture (at ISO 100). Guide numbers are rated at ISO 100; ISO adjustment uses √(ISO/100). Real-world GN may vary from manufacturer specs.
| Flash Type | GN (m, ISO 100) | GN (ft, ISO 100) |
|---|---|---|
| Built-in / Pop-up | 10–14 | 33–46 |
| Small speedlight | 20–30 | 66–98 |
| Mid-range speedlight | 36–43 | 118–141 |
| Pro speedlight | 50–60 | 164–197 |
| Studio strobe (low) | 40–60 | 131–197 |
| Studio strobe (high) | 80–120+ | 262–394+ |
What is a Guide Number? A guide number (GN) is a measure of flash power. It tells you the flash-to-subject distance multiplied by the aperture needed for correct exposure at ISO 100.
Why adjust for ISO? Higher ISO makes the sensor more sensitive, effectively increasing the flash's reach. The GN scales by √(ISO/100).
Are manufacturer GNs accurate? Often optimistic. Test your flash in real conditions for reliable results. Some manufacturers measure at telephoto zoom, which inflates the number.
What about flash modifiers? Softboxes, umbrellas, and diffusers reduce effective GN by 1-2 stops or more depending on the modifier.