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Film speed is the term often used to
describe an emulsion's sensitivity to a given amount of light.
Sensitivity, in turn, is a product of the
amount and configuration of silver halide crystals coated on the film's
emulsion layer. This is true even for color and chromogenic
black-and-white films where the final result is devoid of silver, but
instead is composed of dye clouds. Even these two film types begin with
the exposure of light-sensitive silver,
The difference between ISO and DIN
There are two type of scales being used
to denote film speed.
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ISO which stands for International
Standards Association, the body setting many industrial standards for
precision measurement.
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DIN (a German designation meaning
Deutches Industrie Norm) serves the same purpose as ISO does for the
remainder of the photographic world.
The two scales, ISO and DIN, are
different in one respect.
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ISO is arithmetic; the doubling of an ISO
number indicates an exact doubling of film speed or sensitivity (e.g.,
ISO-200 film is twice as sensitive as an ISO-100 emulsion).
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The DIN system, on the other hand, is not
an arithmetic progression but is instead logarithmic. On the DIN scale,
moving up three numbers denotes a doubling of film speed (e.g., going
from DIN 18 to DIN 21 denotes doubling film speed). The equivalent ISO
numbers are ISO 50 to ISO 100.
Film speed is also denoted by a commonly
accepted verbal classification system:
| Slow |
ISO 6-50 |
DIN 9-18 |
| Medium ISO |
ISO 64-160
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DIN 19-23 |
| Fast ISO |
ISO 200-800 |
DIN 24-30 |
| Ultra-Fast ISO |
ISO 1000-3200
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DIN 31-36
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In addition to the intensity of the light striking a film's surface,
there are several other factors affecting film speed. Many
black-and-white films are not sensitive to all portions of the spectrum
equally and because of this have different EIs. As an example, if you
would choose to use a black-and-white film whose red sensitivity was
low indoors under red-rich tungsten illumination, the film's effective
speed will be lower than its rated speed. In daylight, or using
electronic flash (which is not as red-rich), the normal speed rating
will be restored.
Both black-and-white and a number of color films' speed ratings can be
successfully uprated to provide more than normal sensitivity. An
ISO-100 film can be exposed at ISO 200 or more, often with little
sacrifice in picture quality.
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