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For those spoiled on digital video cameras the concept of film speed for 16 mm film can seem really alien. Just as there is a difference between digital video tapes (though much more minor), there is a difference between 16 mm film stock. Film speed something that is going to be important when considering exactly which type of film you want to use in certain situations.
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Light Sensitivity Measurement
Film speed is a measurement that is used to connote a film stock's sensitivity to light in the area. The lower the sensitivity level the more exposure you will need to get. This makes the film speed slower because the length of film exposure needed is longer. This would end up giving the film stock a lower ISO speed. If a film uses a shorter exposure this makes the ISO speed higher, and is considered to be fast.
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How to Use This
What this means for you is that the ISO speed rating is directly correlated to how much light you need. The faster the film is the more sensitive it is to light and the less you need. The slower the film is, and lower the ISO speed rating, the more light you need. This means that for bright, outdoor events you will likely choose a slower film stock with a lower ISO speed rating. For dark, shaded, or night filming you may want to get a faster film stock with a higher ISO rating. The numbers for this are going to be dependent and it may be best to look at a film speed chart to see what type of film stock you want to use for a given situation. Many film stocks will also be labeled for what type of use they are best for, such as daylight.
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ISO, DIM, and ASA Numbers
The ISO speed rating is likely the one you are going to be using, but there are others as well. The ISO, which stands for International Standards Organization, is one that is used world wide. The ISO number is composed of the DIM number and the ASA number, which is used as the Exposure Index given by the manufacturer so that people have a number to use with their light meter.