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. (
•
0
H-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
