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324 Technical Notes
A Guide Number
To calculate the range of the flash at full power, divide the Guide
Number by the aperture.
If, for example, the flash unit has a Guide
Number of 34 m or 111ft (ISO 100, 20 °C/68 °F); its range at an aperture
of f/5.6 is 34÷5.6 or about 6.1 meters (or in feet,
111÷5.6=approximately 19 ft 10 in.).
For each twofold increase in ISO
sensitivity, multiply the Guide Number by the square root of two
(approximately 1.4).