DX MYSTERIES REVEALEDLooking through a back issue of the CRC News recently I came upon an article by Bill Reid on DX codes (No 90, p13) These are used on some 35mm cassettes to tell cameras and processing machinery something about the film they contain. Bill wanted to know how the coding system worked. Here's how.Camera auto sensing (CAS) code uses a set of electrical probes in appropriately designed 35mm cameras to contact a pattern of conductive or insulating areas on the 135 film cassette. The arrangement of these areas provides an electrically readable encoding of ISO film speed or recommended exposure index, number of exposures on the roll, and recommended exposure latitude.Dimensions and location of the CAS code array on the film cassette is specified in ANSI/NAPM Standard IT1. 14: 1994Holding the magazine with the long hub of the spool to the left and the short hub to the right, and reading from left to right, CAS position numbers 1 to 6 are on top of the CAS code array, and positions 7 to 12 are on the bottom. CAS positions 1 and 7 are always electrically conductive metallic areas CAS positions 2, 3, 4, 5, and 6 encode the ISO film speed or recommended exposure index as follows:|-------------------------------------| | | |----| |----|----|----|----|----|----| |-| | | | 1 | 2 | 3 | 4 | 5 | 6 | | | | | |----|----|----|----|----|----| | | | | | 7 | 8 | 9 | 10 | 11 | 12 | | | |----| |----|----|----|----|----|----| |-| | | |-------------------------------------|Film Speed Conductive CAS Positions ---------- ------------------------ 25 ................. 5 40 ................. 5, 6 64 ................. 2, 6 100 ................. 3, 5 125 ................. 3, 6 160 ................. 3, 5, 6 200 ................. 2, 3, 5 320 ................. 2, 3, 5, 6 400 ................. 4, 5 800 ................. 2, 4, 5 1000 ................. 2, 4, 6 1600 ................. 3, 4, 5 3200 ................. 2, 3, 4, 5CAS positions 8, 9, and 10 encode the number of exposures. All lengths other than 12, 20, 24, and 36 exposures are encoded with insulating areas in all three positions regardless of actual number of exposures.Exposures Conductive CAS Positions --------- ------------------------ 12 .............. 8 20 .............. 9 24 .............. 8, 9 36 .............. 10 all others .............. noneCAS positions 11 and 12 encode exposure latitude:Exposure Latitude Conductive CAS Positions ----------------- ------------------------ + or -1/2 stop .......... none (both insulating) + or -1 stop ............ 11 +2 to -1 stop ............ 12 +3 to -1 stop ............ 11, 12ANSI: American National Standards Institute NAPM: National Association of Photographic ManufacturersI'm not sure where credit for this lies, probably with Kodak, but I acknowledge whoever it was.John Pearle 1 June 1999.
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