Flash Calculator
Calculate your flash's maximum distance, required aperture, or Guide Number (GN). Essential for manual flash photography and studio lighting setup.
Mid-range speedlight
Flash Formulas
Complete Guide to Flash Photography
Understanding your flash's Guide Number is the key to mastering manual flash photography. This guide covers the fundamentals of flash exposure calculation and practical shooting techniques.
What is a Guide Number?
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. Guide Numbers are almost exclusively stated at ISO 100 and calculated for a specific unit (Meters or Feet).
How Guide Numbers Work
The Guide Number is the product of the flash-to-subject distance and the aperture (f-stop). This relationship forms the core formula:
GN = Distance × f-stopExample: If your flash has GN 36 (meters), shooting at f/4 gives a maximum distance of 9 meters.
ISO's Effect on Flash Range
Increasing ISO effectively doubles your flash's power every time you double the ISO value. At ISO 400, your effective GN is multiplied by √4 = 2, doubling your flash range. This is crucial for event photographers working in large venues where flash distance is a limiting factor.
Zoom Heads & Light Modifiers
Modern speedlights have zoom heads that concentrate the light beam. A flash might have a GN of 60 at 200mm zoom but only GN 28 at 35mm. Light modifiers like softboxes, umbrellas, and bounce cards dramatically reduce the effective GN but produce softer, more flattering light. Always check your flash's manual for the GN table at different zoom lengths.
Frequently Asked Questions
What is a good Guide Number for a speedlight?
A GN of 36 (meters at ISO 100) is typical for mid-range speedlights. Professional models like the Canon 600EX or Nikon SB-5000 have GN 60+ at full zoom. Entry-level flashes may be around GN 20-28.
Does bouncing the flash change the Guide Number?
Bouncing flash off a ceiling or wall significantly reduces the effective GN because the light travels a longer path and loses energy on each surface reflection. Expect to lose 1-3 stops of light when bouncing.
Why are GN values given in both meters and feet?
Different markets use different measurement systems. A GN of 36 in meters equals GN 118 in feet (multiply by 3.28). Always verify which unit your flash manufacturer uses.
Can I use Guide Number with TTL flash?
TTL (Through-The-Lens) metering automates the flash power calculation. However, understanding GN is still valuable for manual override, off-camera flash, and situations where TTL may be fooled (very bright/dark subjects).