Nikon SB-500 AF Speedlight Users Manual - English - Page 72

Guide Number, Aperture and Flash-to-subject Distance

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Guide Number, Aperture and Flash-to-subject Distance Tips on Speedlight Care and Reference Information 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-500's guide number is 24/78.7 (ISO 100, m/ft, angle of illumination: covers angle of view of 24mm lens, FX format, temperature: 23 ˚C/73.4 ˚F). When ISO sensitivity is 100 and aperture f-number is f/8, the illumination of the SB-500 reaches 3 m (9.8 ft), which is determined by the equation, flash-to-subject distance (3 m or 9.8 ft) = guide number (24/78.7) / aperture f-number (f/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 the guide number table. (0H-19) t Determining aperture and flash-to-subject distance for correct exposure Aperture f-number = guide number (GN for ISO 100; m or ft) H × 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 H-5

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H
-
5
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
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-500’s guide number is 24/78.7 (ISO 100, m/ft, angle of
illumination: covers angle of view of 24mm lens, FX format,
temperature: 23
˚
C/73.4
˚
F). When
ISO sensitivity is 100 and aperture
f-number is f/8, the illumination of the SB-500 reaches 3 m (9.8 ft),
which is determined by the equation, flash-to-subject distance (3 m or
9.8 ft) = guide number (24/78.7) / aperture f-number (f/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 the guide number table. (
0
H-19)
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