An indication of film stock sensitivity or emulsion speed. Defined by the ASA and combined with the ANSI (American National Standards Institute).
Actually defined only for black and white films, but also used in the trade for colour films. ASA is known as an arithmetical system, as the speed figures have a most simple relationship to each other. For example, 160 ASA is half the speed of 320 ASA, and 640 ASA is twice as fast as 320 ASA. The ASA System is now superseded by ISO International Organization for Standardization.