ASA, an acronym for American Standards Association, refers to an antiquated scale for film speeds devised by Kodak during the 1940s. Film speed measures the degree to which film is sensitive to light. For example, a higher film speed indicates that the film requires less exposure to light, i.e. shorter exposure, to capture an image. Conversely, a lower ASA or ISO number means that the film is a “slow film” that needs longer light exposure to capture the same image.
Today, ASA film speed measures have been replaced by the ISO (International Organization for Standardization) standard that includes both linear and logarithmic dimensions.
The ASA and ISO film speed scales are important because they determine photo quality: the graininess of a finished photo. The sequence runs as follows: the larger the film’s grains, the faster the film, the less light exposure needed. Therefore, highly defined and detailed pictures such as portraits are generally taken on finely grained slow film. In contrast, scenes that have limited lighting or are in motion should generally be taken with larger-grained fast film that can capture an image with minimal light exposure.
Different film companies (such as Canon, Fuji and Kodak) produce distinctly different film speeds. Choose a film with a suitable speed based on the type of pictures you plan on taking.
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