Nikon SB-910 - Manual (Page 115)

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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 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) × aperture f-number. The SB-910’s guide
number is 34 m (111.5 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 aperture f-number is 8, the illumination of the SB-910 reaches 4.25 m
(13.9 ft), which is determined by the equation, fl ash-to-subject distance (4.25 m or
13.9 ft) = guide number (34 m or 111.5 ft) / aperture f-number (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-18)
t
Determining aperture and fl ash-to-subject distance for correct
exposure
Aperture f-number
= 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 / aperture f-number