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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
eects.
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.