Flash Level (Guide Number)

In flash photography, the light from the flash is used to illuminate the subject. The distance at which a built-in flash can light a subject is however limited. The flash guide number (GN) is a measure of the distance at which the flash can illuminate a subject. The higher the guide number, the greater the distance at which the light from the flash is sufficient for optimal exposure. The formula for calculating the guide number is as follows:

Guide number (GN)=distance (meters) × aperture (f-number)

Using the guide number, it is easy to calculate the how close the subject should be at a given aperture or the aperture required to photograph a subject at a given distance. If the flash unit has a guide number of 12 at ISO 100 and aperture is set to f/4, the subject can be up to 3 m away. If the subject is more than 3 m distant , the flash will not be bright enough to light the subject and the photograph will be underexposed. Increasing ISO sensitivity increases the guide number; consequently, raising ISO sensitivity makes it possible to achieve optimal exposure at greater distances. Digital SLR cameras can also be used with optional flash units with higher guide numbers. Using an optional flash unit such as the Nikon SB-400 or SB-900 makes it possible to achieve optimal exposure at greater distances than with the built-in flash. We recommend using an optional flash unit when taking photographs at weddings and in other situations in which you may frequently be taking photographs at a distance from the subject.

Subject photographed
at short range with a low-power (low guide number) flash

Subject photographed
at long range with a low-power (low guide number) flash

Subject photographed
at long range with a high-power (high guide number) flash

Illustrations are an artist’s conception.

Photographs taken at different distances with a built-in flash

Subject close to camera

Subject far from camera

These photographs were taken using a built-in flash with the subject at different distances. At fairly close distances, the light from the flash is sufficient for optimal exposure, but the farther the subject is from the camera the darker the photographs become.

Photographs taken at different ISO sensitivities

ISO 200

ISO 1600

It may sometimes be possible to achieve optimal exposure with distant subjects by raising ISO sensitivity. These photographs, taken with built-in flashes with the same guide numbers, show that raising ISO sensitivity allows subjects to be photographed at a greater distance.

Optional Flash Units

Digital SLR cameras can be used with optional flash units with guide numbers higher than the built-in flash. These optional flash units are more powerful than the built-in flash. The light from these flash units will reach subjects at a greater distance, ensuring that they are correctly exposed.