noun
: a number characteristic of a given photoflash lamp and a given film speed that when divided by the distance in feet between the lamp and the subject indicates the correct f-number for that distance
noun
: a number characteristic of a given photoflash lamp and a given film speed that when divided by the distance in feet between the lamp and the subject indicates the correct f-number for that distance
Webster's New International English Dictionary. Новый международный словарь английского языка Webster. 2012