Here's some explanation about DX codes:
CAMERA AUTO SENSING (CAS) CODES
Camera auto sensing (CAS) code utilizes a set of electrical probes in
appropriately designed 35mm cameras to contact a pattern of conductive or
insulationg areas on the 135 film magazine. The arrangement of these areas
provides an electrically readable encodement 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 magazine 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 through 6 are on top of the CAS code array, and positions 7
through 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
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