Guide Number, Aperture and Flash-to-subject Distance

Tips on Speedlight Care and Reference Information

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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)

tDetermining 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

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