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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

The model number of a flash refers to the flash’s power, and the maximum 
amount of light created is indicated 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.

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.