DX MYSTERIES REVEALED 
  

Looking 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: 1994

Holding 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, 5

CAS 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 .............. none

CAS 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, 12

ANSI:  American National Standards Institute
NAPM:  National Association of Photographic Manufacturers

I'm not sure where credit for this lies, probably with Kodak, but I acknowledge
whoever it was.

John Pearle 
1 June 1999.

Some thoughts on Weighing and Mixing

Editorial

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