To start a discussion on flash photography, the investigator must first understand the concept of guide numbers. A guide number is a value offered by manufacturers that indicates the amount of power or light output a particular flash unit possesses. Guide number values are typically offered in relation to ISO 100 speed film or a digital camera’s ISO setting. The value of a guide number is not arbitrary. A guide number is the product of the f/number (size of an aperture) and the distance between the flash and subject. The formula can be adjusted so that if two of the variables are known the third variable can be calculated. The three formulas for guide numbers and their variables are as follows:
• Guide Number = Aperture × Subject Distance • Aperture = Guide Number/Subject Distance • Subject Distance = Guide Number/Aperture
Manufacturers may occasionally provide additional values and information based on alternative ISO values and subject-to-flash distances based on the metric system. Furthermore, guide number values can change (have different values) for the same flash unit based on the flash head’s zoom setting. A flash’s zoom is designed to adjust the produced light’s field
of view or coverage to match the focal length of the lens mounted to the camera. Widerangle lenses will tell the flash to allow the light to be more dispersed, while longer focal length lenses will communicate to the flash that the produced light should be focused more tightly. Typically, the longer focal lengths or zoom settings will provide higher guide number values.