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Flash
Guide Numbers for Determ ining the Correct Aperture
Guide numbers help you determine a correct exposure or proper aperture (f/stop) when
using the SB-28D in the Manual or Repeating Flash mode.
The guide number represents the amount of light at ISO 200 for meters/feet (m/ft)
generated by the flash. With the SB-28D, you can calculate a correct aperture or desired
flash shooting distance by using the following equations and the guide number table.

Calculating the Correct Apertu r e

Calculating the Shooting Distance

Guide Number Table

Guide Numbers in meters/feet at ISO 200 in Manual and Repeating Flash modes.
Flash
Output
Level
Zoom-head position
18mm 20mm 24mm 28mm 35mm 50mm 70mm 85mm
1/1 (full) 25/84 28/93 42/139 45/148 51/167 59/195 68/223 71/232
1/2 18/59 20/66 30/98 32/105 36/118 42/138 48/157 50/164
1/4 12.7/42 14/46 21/69 22.5/74 25.5/84 30/98 34/112 36/118
1/8 9/30 10/33 15/49 16/53 18/59 21/69 24/79 25/82
1/16 6.4/21 7/23 10.5/35 11.3/37 12.7/42 15/49 17/56 18/59
1/32 4.5/15 5/16 7.5/25 8/26 9/30 10.5/35 12/39 12.7/42
1/64 3.2/10 3.5/11 5.3/17 5.7/19 6.4/21 7.5/25 8.5/28 9/30
M
f/stop (aperture) = Guide number
Flash shooting distance (m/ft)
flash shooting distance (m/ft) = Guide number
f/stop (aperture)