Samsung NX1 user manual Flash guide number

Concepts in Photography

Flash

Light is one of the most important components in photography. However, you will not always have enough of light in all situations. Making use of a flash allows you to compensate for the lack of light and create a variety of effects.

The flash, also known as a strobe or speed light, provides adequate exposure in low-light conditions. It is also useful in light-abundant situations. For example, you can use the flash to provide light when a subject is in shadow or use it to capture clearly both the subject and the background under backlit conditions.

Back lit subjects without flash Back lit subjects with flash

Flash guide number

The model number of a flash refers to the flash’s power, and the maximum amount of light created is represented by a value known as the “guide number“. A flash with a higher guide number emits a larger amount

of light than a flash with a lower guide number. The guide number is calculated by multiplying the distance from the flash to the subject by the aperture value when the ISO sensitivity is set to 100.

Guide number = Flash to Subject Distance X Aperture value

Aperture value = Guide number/Flash to Subject Distance

Flash to Subject Distance = Guide number/Aperture value

If you know the flash’s guide number, you can estimate an appropriate flash-to-subject distance when setting the flash manually. For example, if a flash has a guide number of GN 20 and is 4 meters away from the subject, the optimal aperture value is F5.0.

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