H-6
H
Tips on Speedlight Care and Reference Information
En-01
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-5000’s
guide number is 34.5/113 (ISO 100, m/ft, zoom head position: 35 mm, FX
format, illumination pattern: standard, temperature: 23 °C/73.4 °F). When
ISO sensitivity is 100 and aperture f-number is f/8, the illumination of the
SB-5000 reaches 4.31 m (14 ft), which is determined by the equation, flash-
to-subject distance (4.31 m or 14 ft) ≈ guide number (34.5/113) / 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 (
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
