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H–6
Tips on Speedlight Care and Reference Information
H
Guide Number, Aperture and Flash-to-subject Distance
The guide number (GN) indicates the amount of light generated by a
fl ash unit. As the number increases, the fl ash output becomes greater
and the light extends further.
There is a relation represented by an equation, guide number (m or
ft.; for ISO 100) = fl ash-to-subject distance (m or ft.) × f-number of
aperture. The SB-700’s guide number is 28 m (92 ft.) (for ISO 100,
zoom head position: 35 mm, FX format, illumination pattern: standard,
temperature: 20 ˚C/68 ˚F). When ISO sensitivity is 100 and f-number
of aperture is 8, the illumination of the SB-700 reaches 3.5 m (11 ft.),
which is determined by the equation, fl ash-to-subject distance (3.5 m
or 11 ft.) = guide number (28 m or 92 ft.) / f-number of aperture (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 more details. (• 0H-21)
t
Determining aperture and fl ash-to-subject distance for
correct exposure
f-number of aperture =
guide number (GN for ISO 100; m or ft.) ×
ISO sensitivity factor / fl ash-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 / f-number of aperture