Appendix N

ASA/ISO Numbers

Four interrelated numbering systems are commonly used to rate the speed of various films. This can be confusing if you don't understand how these numbers correlate with one another, but it's really very simple.

ASA numbers increase geometrically as the film speed increases. As the speed doubles, the ASA number also doubles. When compared to a film rated ASA 200, a film rated ASA 400 requires one-half the exposure (one f/stop less exposure) to produce a given density. Because of this logical relationship, and because ASA is still in common usage, I use ASA numbers as a reference for film speed throughout this book. ASA stands for the American Standards Association, now known as the American National Standards Institute.

DIN numbers are the European standard for film-speed rating. DIN numbers progress logarithmically. As the film speed doubles, the DIN number increases by three. ISO numbers combine ASA and DIN into one number. For example, Tri-X is rated ASA 400, DIN 27, and ISO 400/27.

EI stands for Exposure Index. These numbers are used in the same way as ASA numbers but indicate that the user determined the film speed through a testing procedure.

0 0

Post a comment