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Flash
Guide Numbers for Determining the Correct Aperture
Guide numbers help you determine a correct exposure or proper aperture (f/stop) when
using the SB-28D Speedlight 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 Speedlight, you can calculate a correct aperture
or desired flash shooting distance by using the following equations and the guide number
table.
Calculating the Correct Aperture
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
18 mm 20 mm 24 mm 28 mm 35 mm 50 mm 70 mm 85 mm
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)