Nikon SB-24 Guide Number Chart

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

Power 24mm 28mm 35mm 50mm 70mm 85mm
1/1 30 (98) 22 (105) 36 (118) 42 (138) 47 (154) 50 (164)
1/2 21 (69) 22 (72) 25 (82) 30 (98) 33 (108) 36 (118)
1/4 15 (49) 16 (52) 18 (59) 21 (69) 23 (75) 25 (82)
1/8 10.5 (34) 11 (36) 12.5 (41) 15 (49) 16.5 (54) 18 (59)
1/16 7.5 (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

LCD Screen and Controls

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

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.

Power Guide Number / 3m = f/stop
1/1 50m f/16.7
1/2 36m f/12
1/4 25m f/8.3
1/8 18m f/6
1/16 12.5 f/4.2