Guide Number / Flash Performance

The guide number is the product of the diaphragms f-number and the distance to subject for correct exposure. The number indicates the flash range and therefore the flash’s performance. In order to compare flash devices properly, the aspects mentioned below should be taken into consideration.

Formula: Gn= distance x f-number

Gn (Guide number)
Distance (meter)

Example calculation:
A flash with a guide number of 40 with an ISO rate of 100 and a diaphragm of f/8 creates sufficient illumination for a distance of 5 meters.

Thanks to modern electronic flash devices, fully developed TTL metering and also the option to check images on digital cameras, you are rarely required to calculate flash and camera settings nowadays. However, this formula offers insight into the performance capability of flashes.  

For an optimal comparison of flash performance, the value should correlate with ISO 100. Because this specification is not standardised, you should make sure when purchasing a flash device which ISO rate the guide number specification applies to and if need be convert it. If the specification applies to ISO 200, the guide number must be divided to result in the quotient 1.4 (precise value √2), in order for the value to correlate with ISO 100. If the guide number applies to ISO 50 (seldom), the guide number’s value must be multiplied with the factor 1.4 (precise value √2) in order to allow comparison on the basis of ISO 100.

The reflector’s beam angle (zoom setting) has a deciding influence on the flash’s range and guide number. Most of the modern compact flash devices adapt automatically to the beam angle of the flash’s focal length and therefore optimise the flash’s range.  In order to compare flash devices performance, the manufacturer’s information about guide number for the same focal length/same beam angle of the flash reflector must also be analysed.

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