film speed

photography The ASA, DIN, and other ratings on films indicate the rapidity of response of the photographic emulsion to light. All have values increasing with speed, the ASA values being linearly proportional to speed.

The DIN and Scheiner schemes are exponential, expressed in degrees with 6° representing a quadrupling of speed; but the British Scheiner scheme has values 10 above the DIN, the American Scheiner 5 above DIN. The alphabetical Ilford scheme is also exponential, with successive letters being doublings (upward from A). The first widely used scheme was the H and D, which was identical to the ASA except for being multiplied by 25. Table 15 shows comparative values, but the method of assessing differs: ASA and H and D indicate maximal sensitivity, DIN and Scheiner the minimal exposure for forming a detectable image.