Basics Photography: Light (p90-91)

• A modern flashgun is a most versatile light source - light can be bounced or diffused but exposure is only correct for a set distance.
• There are three key specifications for any flashgun – guide number (a measure of flash power for a given film speed), recycling time (the speed to recharge between flashes) and coverage (the angle that the flash beam covers).
• Guide number = aperture x distance - to work out the aperture: measure the distance to the subject (use your lens scale). Divide the guide number by this distance to get the f-stop.
• To check the guide number of an unknown flash unit: set the unit to manual and the flash meter for ISO 100, place the flash 1 metre distant and take a flash reading. The aperture is the guide number.