26Concepts in PhotographyFlash guide numberThe model number of a flash refers to the flash’s power, and the maximumamount of light created is represented by a value known as a "guidenumber." The bigger the guide number, the more light is emitted from theflash. The guide number is achieved by multiplying the distance from theflash to the subject and the aperture value when the ISO sensitivity is setto 100.Guide number = Flash to Subject Distance X Aperture valueAperture value = Guide number/Flash to Subject DistanceFlash to Subject Distance = Guide number/Aperture valueTherefore, if you know the guide number of a flash, you can estimate anoptimum flash to subject distance when setting the flash manually. Forexample, if a flash has a guide number of GN 20 and is 4 meters away fromthe subject, the optimal aperture value is F5.0.FlashLight is one of the most important components in photography. It is noteasy, however, to have an enough amount of light anytime and anywhere.Making use of a flash allows you to optimize light settings and create avariety of effects.Flash, also known as strobe or speed light, helps to create adequateexposure in low-light conditions. It is also useful in light-abundantsituations. For example, flash can be used in compensating the exposureof a subject’s shadow or capturing clearly both the subject and thebackground in backlit conditions.Before correction After correction
