Guide numbers for determining correct aperture
46
The “guide number” represents the amount of light
generated by the flash. With the SB-27 in Manual (M) Flash
mode, you can calculate a correct aperture (f/stop) value by
using the following equation and the “Guide number” table.
—For Guide numbers in 1/300 TTL High-Speed Sync Flash
operation with the F5 camera, see page 40.
To calculate a correct aperture:
f/stop (aperture)=
guide number
flash-to-subject distance (m)
To calculate the shooting distance:
flash-to-subject distance (m)=
guide number
f/stop (aperture)
Guide number (at ISO 100: for m/ft) (at 20˚C/68˚F)
Zoom-head position
Light
24mm
28mm
35mm 50mm
Output
(normal)
1/1 (full) 25/82 27/89 30/98 34/112
1/2 17.7/58 19/62 21.2/69 24/79
1/4 12.5/41 13.5/44 15/49 17/56
1/8 8.8/29 9.5/31 10.5/34 12/39
1/16 6.2/20 6.7/22 7.4/24 8.5/28
The guide number varies with the film speed. For films other than ISO 100,
multiply the above figures by the factors shown in the table below.
Adjustment factors for other ISO film speeds
ISO film speed 25 50 200 400 800 1600
Factor × 0.5 × 0.71 × 1.4 × 2 × 2.8 × 4
For example, when shooting a subject under the following conditions,
—Light output: 1/1 (full),
—Flash mode: Manual (M) ,
—Film speed: ISO 400,
—Zoom-head position: 35mm,
you will obtain a guide number of 60 (30 × 2) using the above table.
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