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Tips on Speedlight Care and Reference Information
Guide Number, Aperture and Flash-to-subject Distance
The guide number (GN) indicates the amount of light generated by a
flash unit. As the number increases, the flash output becomes greater
and the light extends further.
There is a relation represented by an equation, guide number (ISO 100,
m/ft) = flash-to-subject distance (m or ft) × aperture f-number.
The SB-500’s guide number is 24/78.7 (ISO 100, m/ft, angle of
illumination: covers angle of view of 24mm lens, FX format,
temperature: 23 ˚C/73.4 ˚F). When ISO sensitivity is 100 and aperture
f-number is f/8, the illumination of the SB-500 reaches 3 m (9.8 ft),
which is determined by the equation, flash-to-subject distance (3 m or
9.8 ft) = guide number (24/78.7) / aperture f-number (f/8).
• For ISO sensitivities other than 100, multiply the guide number by
the factors ( ISO sensitivity factors) shown in the table below.
ISO 25 50 100 200 400 800 1600 3200 6400
Factor 0.5 0.71 1 1.4 2 2.8 4 5.6 8
• See “Specifications” for the guide number table. (0H-19)
t Determining aperture and flash-to-subject distance
for correct exposure
Aperture f-number
= guide number (GN for ISO 100; m or ft)
× ISO sensitivity factor / flash-to-subject distance (m or ft)
Flash-to-subject distance (m or ft)
= guide number (GN for ISO 100; m or ft)
× ISO sensitivity factor / aperture f-number