Определение и Значение "guide number"

What is the "guide number"?

The guide number is a measure of a flash unit's power and its ability to illuminate a scene. It indicates the maximum distance at which the flash can effectively light a subject, given a specific aperture setting. The guide number is typically expressed in meters or feet and helps photographers determine the appropriate exposure settings for their shots. A higher guide number means the flash has greater power and can cover a larger area or reach subjects farther away. This measurement is useful for adjusting flash output and ensuring proper exposure in various lighting conditions.