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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
ash allows you to compensate for the lack of light and create a variety of
eects.
The ash, 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 ash 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 ash Back lit subjects with ash
Flash guide number
The model number of a ash refers to the ash’s power, and the maximum
amount of light created is represented by a value known as the “guide
number“. A ash with a higher guide number emits a larger amount
of light than a ash with a lower guide number. The guide number is
calculated by multiplying the distance from the ash 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 ash’s guide number, you can estimate an appropriate
ash-to-subject distance when setting the ash manually. For example, if a
ash has a guide number of GN 20 and is 4 meters away from the subject,
the optimal aperture value is F5.0.