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Guide numbers help you determine a correct exposure or proper aperture
(f/stop) when using the SB-28 in the Manual ƒ or Repeating Flash " mode.
• See page 51 for % guide numbers.
The guide number represents the amount of light (ISO 100: for m/ft) generated by
the flash. With the SB-28, you can calculate a correct aperture by using the following
equation and the guide number table.
To calculate the correct aperture:
Guide number (GN)
f/stop (aperture) = ——————————————
Flash shooting distance (m/ft)
To calculate the shooting distance:
Guide number (GN)
Flash shooting distance (m/ft) = ——————————
f/stop (aperture)
• Guide number varies with film speed.
For example, when shooting a subject located 9m (approx. 30 ft) away at 1/1 (full)
flash output, with the zoom-head position at 35mm and a film speed of ISO 100,
first read the guide number from the table above. In this case, it is 36 (or 118).
Then divide the guide number by the shooting distance:
f/stop = 36/9 (in meters) = 4
f/stop = 118/30 (in feet) = 3.93 = approx. 4
Therefore f/4 is the correct aperture.
Guide Numbers for determining correct aperture in
Flash output
level
1/1 (full)
1/2
1/4
1/8
1/16
Zoom-head position
24mm 28mm 35mm 50mm 70mm 85mm
30/98 32/105 36/118 42/138 48/157 50/164
21/69 22.5/74 25.5/84 30/98 34/112 36/118
15/49 16/53 18/59 21/69 24/79 25/82
10.5/35 11.3/37 12.7/42 15/49 17/56 18/59
7.5/25
20mm
20/66
14/46
10/33
7/23
5/16
18mm
18/59
12.7/42
9/30
6.4/21
4.5/15 8/26 9/30 10.5/35 12/39 12.7/42
1/32
1/64
5.3/173.5/113.2/10 5.7/19 6.4/21 7.5/25 8.5/28 9/30
3.8/132.5/82.3/8 4/13 4.5/15 5.3/17 6.0/20 6.3/21
Guide numbers (at ISO 100: for m/ft at 20 C/68 F) in ƒ and " modes