Definitions for "Flash range"
The distance needed to be covered by the flash to give the subject optimal illumination. This can be calculated using Lambert's law. As the distance from the light source is doubled only a quarter of the original amount of light will reach the subject. (Guide number)
The maximum distance from which a flash can effectively illuminate a subject. Most built-in flashes are effective to about 12-15 feet. Range varies by brand, so check the specification carefully.
The maximum range at which the flash illuminates a subject. This can be calculated using Lambert's law. As the distance from the light source is doubled only a quarter of the original amount of light will reach the subject.