H–4
H
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
