20 How to calculate the range of a Flash from the Guide Number.
The output power of a flash is usually stated as the "Guide Number" As the effective range of the flash varies with the camera aperture and the film speed, both of these factors must be taken into account when calculating the distance at which the subject will be adequately illuminated.
For a sensitivity of ISO100, the flash range can be calculated by the following formula:
Flash Range = Guide Number/Aperture.
Therefore a flash with a Guide Number of 8 will have a range of 2 metres if the camera aperture is set to f4.
Note that Guide numbers are usually given in both metric and imperial units (metres and feet).
If a film speed or sensitivity other than ISO100 is used, the flash range will vary by a simple ratio. A higher sensitivity will give a greater range and a lower sensitivity will reduce the flash range. The following table gives a conversion factor which will enable the compensated Guide Number to be calculated from a given film speed or sensitivity.
Students of mathematics will note that the compensation factor is simply the square root of the ratio of the new ISO number to ISO 100.
Film Speed or Sensitivity Conversion Factor ISO 25 0.5 ISO 50 0.71 ISO 100 1 ISO 200 1.4 ISO 400 2 ISO 800 2.8 ISO 1600 4