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+ |
Scenario: Portrait with speedlight at f/5.6, ISO 400
Flash: GN 36 (meters) at ISO 100
Calculation: Effective GN = 36 × √(400/100) = 36 × 2 = 72 meters
Maximum distance: 72 ÷ 5.6 = 12.9 meters at f/5.6
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.
Higher ISO makes the sensor more sensitive, effectively increasing the flash's reach. The GN scales by √(ISO/100).
Often optimistic. Test your flash in real conditions for reliable results. Some manufacturers measure at telephoto zoom, which inflates the number.
Softboxes, umbrellas, and diffusers reduce effective GN by 1-2 stops or more depending on the modifier.
Set your camera to manual mode, choose your aperture and ISO, then adjust flash power until you get proper exposure at the calculated distance.