categories: slow speed (ISO 25 to 32), medium speed (ISO 50 to 125), and
high speed (ISO 160 to 400).
c. Higher speed films of more than ISO 400 are attainable on the market
today which are ultra high speed films.
d. A doubling of the ISO number indicates a doubling of the sensitivity
of the film. Thus, a film with an ISO of 400 rating needs only one half the
amount of light for proper exposure than a film rated at ISO 200.
Conversely, a film rated at ISO of 100 is half as fast or sensitive as a
film rated at ISO 200 and requires twice the amount of light for a proper
exposure.
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