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Calculate depth of field from aperture, focal length, sensor size, and subject distance. See near and far focus limits for sharp photos.
Calculate correct exposure with the exposure triangle. Set ISO, aperture, and shutter speed for balanced photos in any lighting situation.
Convert focal length between camera sensor sizes using crop factor. Find full-frame equivalent focal length for APS-C, MFT, and more.
At the guide number's rated ISO, aperture times distance equals GN. Distance is typically in feet or meters depending on manufacturer rating.
GN = f-number × distanceGN scales with ISO. Doubling ISO increases effective GN by √2 (one stop). Adjust when shooting above or below the flash's rated ISO (usually 100).
GN₂ = GN₁ × √(ISO₂ / ISO₁)Flash falloff follows inverse square law — doubling distance requires 4× power or +2 stops aperture. GN math embeds this relationship.
f = GN / distance; distance = GN / fUpdated: July 2026
At 10 feet, correct aperture is f/6 (GN 60 ÷ 10). At 20 feet, open to f/2.8 or raise ISO by two stops.
Bounce doubles or triples effective path length. If ceiling bounce path is ~15 feet vs direct 8 feet, compensate by ~1 stop narrower aperture or higher ISO.
At f/8 and 10 feet, exposure aligns with GN 80. Use the calculator to verify before adding modifiers that reduce effective power.
Measure total light path from flash to ceiling to subject. Bounce adds distance and scatter loss — open aperture or raise ISO accordingly.
Convert units consistently. GN in feet × distance in feet, or GN in meters × distance in meters. Mixing units produces wrong aperture by a factor of ~3.
Guide number (GN) relates flash power, aperture, and distance for manual exposure. Enter any two values — GN, f-stop, or distance — to solve for the third, with ISO adjustment for non-base sensitivities.