Guide Number (GN) is a numerical rating system that indicates the power output of a camera flash unit. It provides a standardized way to measure and compare flash power across different flash units, helping photographers determine the correct exposure settings when using flash photography.
The Guide Number represents the relationship between aperture, flash-to-subject distance, and ISO sensitivity. It’s typically expressed as a distance measurement (usually in feet or meters) at a specific ISO setting, most commonly ISO 100. A higher Guide Number indicates a more powerful flash that can effectively illuminate subjects at greater distances or allow for smaller apertures.
How Guide Number Works
The Guide Number formula is elegantly simple: GN = Distance × Aperture (f-stop). This means if you know two of these values, you can calculate the third. For example, if your flash has a Guide Number of 40 (meters, ISO 100) and you’re shooting a subject 5 meters away, you would divide 40 by 5 to get f/8 as your appropriate aperture setting.
In practical use, if you’re photographing a portrait subject 10 feet away with a flash rated at GN 80 (feet, ISO 100), you’d use an aperture of f/8 (80 ÷ 10 = 8). If you moved closer to 5 feet, you’d need to close down to f/16 to maintain proper exposure. Conversely, if you increased your ISO from 100 to 400, the effective Guide Number would double, allowing you more flexibility with aperture and distance.
Practical Applications
Guide Numbers are particularly useful when working with manual flash settings or when using off-camera flash setups. While modern TTL (Through-The-Lens) flash metering has reduced the need for manual calculations, understanding Guide Numbers helps photographers make informed decisions about flash power, especially in challenging lighting situations or when mixing ambient light with flash.
When comparing flash units, Guide Numbers provide an objective measure of power output. A speedlight with a GN of 58 meters is significantly more powerful than one rated at GN 36 meters. This difference becomes crucial when shooting in large spaces, using light modifiers that reduce effective power, or when you need maximum reach for distant subjects.
Remember that Guide Numbers assume direct flash hitting the subject. When using diffusers, softboxes, or bounce flash techniques, the effective power is reduced, often by one to three stops depending on the modifier and setup.