24Concepts in PhotographyFlashLight is one of the most important components in photography. It’snot easy, however, to have an enough amount of light anytime andanywhere. Making use of a flash allows you to optimize light settingsand create a variety 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 theexposure of a subject’s shadow or capturing clearly both the subjectand the background in backlit conditions.Before correction After correctionFlash guide numberThe model number of a flash refers to the flash’s power, and themaximum amount of light created is represented by a value knownas a “guide number.” The bigger the guide number, the more light isemitted from the flash. The guide number is achieved by multiplying thedistance from the flash to the subject and the aperture value when theISO sensitivity is set to 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 awayfrom the subject, the optimal aperture value is F5.0.
