Each flash unit has a "GUIDE NUMBER" which describes the amount of light produced. The question answered by the GUIDE NUMBER is "What aperture (f/stop) must I use to illuminate an object at a specified distance?"

To determine the f/stop needed, divide the GUIDE NUMBER by the desired distance.
- or -
To determine the distance that can be illuminated, divide the GUIDE NUMBER by the selected f/stop.

Manufacturers usually specify a GUIDE NUMBER for ISO 100 speed film. Faster film will react to less light, allowing proper exposure at greater distances, thus necessitating a different GUIDE NUMBER for each film speed. As can be seen from the table below, increasing the ISO film speed by a factor of 4 will double the GUIDE NUMBER. The GUIDE NUMBERs for the Minox flash units at the left are given, in the descriptive material for the meters, for a film speed of ISO 100. The GUIDE NUMBER for film speeds other than those provided in the meter documentation may be computed by a technique like one shown in the table below. Note in the illustration at the left for the Flash 8x11, that a GUIDE NUMBER of 12, when using meters as a unit of measure, is equivalent to a GUIDE NUMBER of 39 when using feet as a unit of measure. Be sure you know upon which units of measure the GUIDE NUMBER is based!

If the GUIDE NUMBER is known for a specific film speed, a Guide Factor may be calculated by dividing the known GUIDE NUMBER by the square-root of the known film speed (ISO). This Guide Factor will be the same REGARDLESS of the film speed. A new GUIDE NUMBER may now be computed for any film speed by multiplying the square-root of the new ISO by the Guide Factor.	ISO	Square Root of ISO	Guide Number	Guide Factor
	100	10	39	3.9
	400	20	78	3.9
	64	8	31.2	3.9
	25	5	19.5	3.9