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Concepts in Photography

Flash

Light is one of the most important components in photography. It is not 
easy, however, to have an enough amount of light anytime and anywhere. 
Making use of a flash allows you to optimize light settings and create a 
variety of effects.

Flash, also known as strobe or speed light, helps to create adequate 
exposure in low-light conditions. It is also useful in light-abundant 
situations. For example, flash can be used in compensating the exposure 
of a subject’s shadow or capturing clearly both the subject and the 
background in backlit conditions.

Before correction

After correction

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 a "guide 
number". The bigger the guide number, the more light is emitted from the 
flash. The guide number is achieved by multiplying the distance from the 
flash to the subject and the aperture value when the ISO sensitivity is set 
to 100.

Therefore, if you know the guide number of a flash, you can estimate an 
optimum 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.