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Concepts in Photography
Flash
Light is one of the most important components in photography. It’s
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
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 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.
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
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.