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Catalogue

Aspect ratio (at ISO 100)

The directional number is a value that indicates the power of the photographic flash. Simply put, it tells you how far, in meters, the flash can properly illuminate a scene at a base sensor sensitivity of ISO 100. It is used to quickly manually calculate the correct exposure when shooting with a flash. The relationship between directional number, distance and aperture is key and is calculated as: directional number = distance of subject from flash × aperture number used. So if the flash has a directional number of 40 and the subject is 10 metres away, you need to set the aperture number to f/4 for a correct exposure, as 40 divided by 10 is 4. Conversely, if you want to use an aperture of f/8 with the same flash, your subject can be no more than 5 metres away. The value is always standardized for ISO 100 sensitivity so that flashes can be compared with each other. If you increase the ISO sensitivity on the camera, the effective range of the flash will be extended. Manufacturers often quote the highest possible value of the directional number that the flash achieves at maximum zoom, i.e. "zooming" the flash head when the light is most concentrated. Although modern automated systems such as TTL metering perform these calculations for the photographer, the directional number remains the basic indicator for comparing the actual performance of different flashes.

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