Flash Calculator
Flash Range & Guide Number Calculator
Calculate your flash's maximum distance, required aperture, or Guide Number (GN). Essential for manual flash photography and studio lighting setup.
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Understanding Flash Guide Numbers
A flash's Guide Number (GN) is a universally accepted method of rating its power. It provides a simple calculation allowing manual flash shooters to instantly determine the correct f-stop or the maximum working distance without relying on a light meter or TTL (Through-The-Lens) metering.
Important Standard: Guide Numbers are almost exclusively stated at ISO 100 and calculated for a specific unit (Meters or Feet). It's crucial to know which unit your flash manufacturer is using (e.g., GN 36 in meters = GN 118 in feet).
Zoom Heads Affect GN
Modern speedlights have zoom heads that concentrate the light. A flash might have a GN of 60 at 200mm zoom, but only a GN of 28 at 35mm. Always check your flash's user manual for the GN table at different zoom lengths.
Flash Formulas
Distance Calculation
To find how far your flash can reach, divide its Guide Number by the f-stop you intend to use.
Aperture Calculation
To find the correct f-stop for a subject at a specific distance, divide the GN by that distance.
ISO Adjustment
Increasing ISO effectively increases your flash range. Doubling the ISO increases GN by a factor of 1.4 (√2).
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