Guide Number

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The guide number of a flash provides a simple means for comparing flash power and for estimating the necessary f-stop for proper exposure when using flash. Strangely, I’ve not previously posted about guide numbers, so here goes.

Divide the guide number by the distance to the subject to get the f-stop.

Is that it? How could this possibly work? Imagine setting up a totally manual single flash such that the exposure is correct; the guide number is then defined as
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GN= distance x camera f-stop
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If the distance is in meters, then the GN is expressed in meters. If the distance is in feet, then the GN is expressed in feet. (To convert a GN given in meters to a GN given in feet, multiply the meters GN x 3.28) .

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To further complicate matters, the guide number changes with film/sensor ISO speed. The convention is that ISO 100 is a convenient reference point. If you know the guide number at ISO 100 then the guide number at ISO 400 is 2x the GN at ISO 100 (that is, the square root of the ratio).

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Here are some examples of guide numbers:

Canon G9 built-in flash: GN = (about) 3 m; not specifically provided
Canon 7D built-in flash: GN= 12 m
Canon 580EX: GN=58 m
Canon 420EX: GN = 42m
Canon 220EX: GN = 22 m
Nikon SB-28: GN = 50 m
Nikon SB-24: GN = 36 m
Yongnuo 460-II: GN = 53 m

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Notice that the Canon flash model number is the guide number followed by a zero. For flashes with zoom capability, the manufacturers usually provide the guide number at maximum zoom (probably because this is a larger number). Sometimes tables of guide numbers are provided at various zoom and ISO settings.

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Here's an example of the guide number in use. Suppose that your flash has GN=40 m, your camera is set for ISO 100 and the flash is 5 meters from the subject. Divide 40 by 5 to get 8 – this is the correct f-stop for your camera (i.e., set f8 on the camera).

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The guide number is best used as a first approximation and actual results vary considerably with the location, flash orientation, bounced light, etc., etc.

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Wikipedia and the Strobist have more detailed information on guide numbers.

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