Appendix Q 
ASA/ISO Numbers
Four interrelated numbering systems are commonly used to rate the speed of various films. The numbering can be confusing if you don’t understand how the numbers correlate with one another, but the system is 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 ...
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