GN, short for Guide Number, Is the amount of power the light transmitted by a flash device is. It’s the result from multiplying distance (in meter or feet) and aperture.
The power of the flash is measured with the GN which describes the flash’s capacity in illuminating objects within a certain distance (in meters). Modern flashes supports TLL technology that allows it to set its power according to the distance the object is to the camera, so the risk of it being too bright or too dark can be eliminated. Read the rest of this entry » .