Digital Camera guide number, indicator of the flash strength

These calculations are simplified with the adoption of a rule that served as a fast calculation method for the aperture and an indicator of the flash strength: the guide number (GN). This number is associated with the flash and is precisely defined in the previous example: it is the necessary aperture to be able to correctly expose an ISO 100 emulsion when the theme is a surface lightened by flash at a meter of distance. With this number it is very easy to know the aperture we need for each distance: it's only necessary to divide the GN by the number of meters .

The number obtained is that of the aperture we need (see previous example). Each flash comes with an aperture table and distances that is nothing but a representation of the guide number rule for different distance and sensibilities. The greater the guide number, the more potent the flash is. A large guide number implies the use of more closed apertures at all distances, greater reach in light distance, and greater possibilities of using the flash as a source of soft, bounces illumination. The inconvenience of these high strength devises are: it's weight and volume, and its energy consumption.