Nikon D700 User Manual page 405

• Guide Number
To calculate the range of the flash at full power, divide the Guide Number
by the aperture. For example, at ISO 100 the 58-800 has a Guide Number
of 38 m or 125 ft. (35 mm zoom head position); its range at an aperture of
f/5.6 is 38+5.6 or about 6.8 meters (or in feet, 125+5.6=approximately
23 ft. 7 in.). For each twofold increase in 150 sensitivity, multiply the Guide
Number by the square root of two (approximately 1.4).
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