36 ABSOLUTE BEGINNER’S GUIDE TODIGITAL PHOTOGRAPHY
FIGURE 3.2
ISO 100-, ISO
400-, and ISO
1600-speed film
compared.Film Speed Rating Systems
A film speed number indicates how sensitive that film is to light. There are several
rating systems for film speed. The most common in English-speaking countries are
ISO (International Organization for Standardization) and EI (exposure index). ASA
(American Standards Association) is an older rating system. All use the same numer-
ical progression: The film speed rating doubles each time the light sensitivity of the
film doubles.