Guide number
Photopedia
23 October 2007 12:20
The guide number (GN) is probably the most useful and important specification to know about an electronic flash unit, as it quantifies the amount of light a particular flash can produce.
The higher the GN, the more light the flash can emit but it is important to check how the GN is expressed, since the information will vary considerably depending on the following factors: the sensitivity (ISO rating), the units of distance (feet or meters), the angle of coverage of the flash tube (usually given as an equivalent to a lens focal length).
The use of any attachments that modify the path of light from the flash tube will also affect the GN rating of the unit.
Typically a Guide Number is expressed in the following terms:
GN = X feet (ISO100, 35mm), or GN= X metres (ISO100, 35mm).
For example, GN = 32 m (ISO100, 35mm). In this case using a sensitivity of ISO 100, and an angle-of-view equivalent to a lens with a focal length of 35mm on the 135-film (35mm) format, the GN = 32.
If you know two of the three following values—GN, lens aperture, and flash-to-subject distance—you can calculate the third value.
• Guide Number (GN) = Aperture (f) x Distance (m/ft)
• Aperture (f) = Guide Number (GN) / Distance (m/ft)
• Distance (m/ft) = Guide Number (GN) / Aperture (f)
Using the example above, with a lens aperture of f/8 the flash will provide correct exposure at a subject-to-flash distance of 4m (4 = 32/8). It is important to make sure you use the same units of measurement throughout any calculations.