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
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.
| 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 |