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Concepts in Photography
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 metres away from the subject, the optimal aperture value is F5.0.
Bounce photography
Bounce photography refers to the method of bouncing light off of the ceiling or walls so
that the light spreads evenly over the subject. Normally, photos taken with flash may appear
unnatural and cast shadows. Subjects in photos taken with bounce photography cast no
shadows and look smooth due to evenly spread light.
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