Photography - Flash Guide numbers - Electronic Flash Information
The further away the subject is from the flash unit, the more energy is needed to give sufficient illumination for proper exposure. If the power or the flash stays constant, it is necessary to open up the camera’s aperture as the distance between the flash unit and the subject is increased. The distance and the aperture value for proper exposure are connected by this formula:
subject distance * f-stop value = constant = GN
This constant is called the Guide Number (GN for short). Since f-stop values have no units attached, the guide numbers are expressed in units of length (feet in the U.S., meters most elsewhere). Guide numbers are valid only for a given speed of film, and typically are expressed for ISO 100.
In order to double the guide number, four times more energy needs to be stored in the capacitor. This puts practical limits to guide numbers, since the size, weight, and cost of the capacitor increase as its capacity is increased. In practice, most flash units built into cameras are limited to about 15m/45ft or even less, while off-camera units are typically in the 24-50m/80-150ft range, with some pro models slightly higher. Studio flash units, which can be powered from AC power and need not be carried around, may have much higher guide numbers.