By altering the size and composition of the silver grains in the emulsion, films can be made more or less sensitive to light. The more sensitive an emulsion is, the less light it takes to produce a given density. Very sensitive films are said to be faster than films that are less sensitive because they allow you to use a faster shutter speed in a given lighting situation. Films of various speeds are assigned numbers by the American Standards Association. These are called ASA numbers. The important thing to remember about ASA numbers is that the higher the film's ASA number, the faster the film. Thus a film rated ASA 400 is more sensitive to light than a film rated ASA 125. Faster films tend to be more grainy than slower films, but they're an advantage when you're shooting in low-light situations.
You will learn much more about ASA and its effect on exposure and development in the main body of this book, but the essential concept is as simple as that. See Appendix N.
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