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Dx Encoding DX (Digital indeX) is a standard for marking 135 and APS photographic film cartridges. Cameras can electrically determine the film speed, number of exposures and exposure tolerance. The DX Camera Auto-Sensing Code takes the form of a common ground and ten areas (bits) on the cartridge surface that are either conductive or non-conductive. Electrical contacts in the camera read the bit pattern. The first 5 bits represent 32 possible film speeds. But there are only 24 speeds representing the 1/3 "stops" from 25 ISO to 5000 ISO. Likewise, 3 bits represent possible 8 film lengths, although in practice only 12, 20, 24 and 36 exposures are encoded. Finally, 2 bits give 4 ranges of film tolerance, or latitude, in ±f-stops. The full DX geometrical and electrical standard is set by ANSI and I3A. See also External link - http://www.geocities.com/thombell/dx.html (Shows how to read the codes.)
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