Flash Guide Number; Bounce Photography - Samsung NX3000 User Manual

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 meters 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 captured with flash may appear unnatural and cast shadows.
Subjects in photos captured with bounce photography cast no shadows
and look smooth due to evenly spread light.
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