Strobe Lighting Masterclass
Demystifying Flash Guide Numbers & The Inverse Square Law
Understanding your speedlight or studio strobe's Guide Number ($\text{GN}$) unlocks absolute confidence when shooting manual flash. Instead of guessing power settings, simple physics provides the exact aperture or distance needed to balance ambient background light with flattering subject illumination.
| Flash Power Ratio | Relative Stops Loss | Effective GN Factor | Typical Application |
|---|---|---|---|
| 1/1 (Full Power) | 0 Stops | 1.00× Base GN | Overpowering bright midday sunlight |
| 1/4 Power | -2 Stops | 0.50× Base GN | Outdoor fill flash with rapid recycling |
| 1/16 Power | -4 Stops | 0.25× Base GN | Studio portrait key light with fast primes (f/1.4 - f/2.8) |
| 1/64 Power | -6 Stops | 0.125× Base GN | High-speed action freeze with sub-millisecond flash duration |
What is the Flash Guide Number formula?
The fundamental flash equation is: Guide Number (GN) = Aperture (f-number) × Subject Distance. For example, if a flash has a Guide Number of 60 meters and your aperture is f/4, the maximum distance for proper subject illumination is: 60 / 4 = 15 meters.
How does ISO affect my speedlight's Guide Number?
Guide numbers are standardly rated at ISO 100. Because ISO represents sensor sensitivity, every 2-stop increase in ISO doubles the effective Guide Number (e.g. increasing from ISO 100 to ISO 400 doubles light reach from GN 60 to GN 120!).