Nikon SB-24 Guide Number Chart

By Nathaniel Stephan
Last Updated: August 8, 2019
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Nikon SB-24 Speedlight

The flash guide number is the maximum distance a subject can be away from the flash and still be properly illuminated. This means the guide number can be used as a proxy for how powerful a flash is.

The guide number for the flash is 42m (138ft) with a 50mm lens. More general information on the flash can be found on this Nikon SB-24 Speedlight page.

This chart below was recreated from the one found on page 81 of the Nikon SB-24 manual [PDF].

You can determine the correct f/stop at a given distance with this equation:

f/stop = guide number / flash to subject distance

Power24mm28mm35mm50mm70mm85mm
1/130 (98)22 (105)36 (118)42 (138)47 (154)50 (164)
1/221 (69)22 (72)25 (82)30 (98)33 (108)36 (118)
1/415 (49)16 (52)18 (59)21 (69)23 (75)25 (82)
1/810.5 (34)11 (36)12.5 (41)15 (49)16.5 (54)18 (59)
1/167.5 (25)8 (26)9 (29)10.5 (34)11.5 (38)12.5 (41)

Distances are in meters (ft).
For film speeds different from ISO 100, multiply the distance in by the following:

  • ISO 25 x0.5
  • ISO 50 x0.71
  • ISO 200 x1.4
  • ISO 400 x2
  • ISO 800 x2.8
  • ISO 1600 x4
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Example Calculations

50mm lens

Let's say you wanted to use the flash with a 50mm lens. If the flash is 21m away from the subject, we can calculate the f/stop to use on the lens.

At full power the flash will have a guide number of 42m.

f/stop = 42m / 21m
f/stop = 2

85mm lens

Now let's say you want to use a 85mm lens with the flash 3m away from the subject. By varying the power output of the flash, we can control which f/stop will produce a well exposed image.

PowerGuide Number/ 3m = f/stop
1/150mf/16.7
1/236mf/12
1/425mf/8.3
1/818mf/6
1/1612.5f/4.2

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