DX encoding

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DX (Digital indeX) is a standard for marking 135 and APS photographic film cartridges. Cameras can electronically determine the film speed, number of exposures and exposure tolerance. The first 35mm camera to use the technology was the Konica TC-X, which was introduced in 1985.

The DX Camera Auto-Sensing Code takes the form of a grid of contact points on the side of the cartridge surface that are either conductive or non-conductive. Electrical contacts in the camera read the bit pattern.

[edit] Electrical contacts

On 35mm film canisters there are two rows of six rectangular areas, the two left-most areas (with the spool post on the left) are both common-ground and are thus always bare metal. The remaining 5 bits in the top row represent 32 possible film speeds. But there are only 24 speeds representing the 1/3 stops from 25 ISO to 5000 ISO.

In the second row, the first 3 bits represent 8 possible film lengths, although in practice only 12, 20, 24 and 36 exposures are encoded. The remaining 2 bits of the second row give 4 ranges of exposure tolerance, or latitude.

Diagramatically (with spool post to the left):

G	S1	S2	S3	S4	S5
G	L1	L2	L3	T1	T2

Where 'G' are the two common-ground contacts, 'S' is the film speed, 'L' the film length, and 'T' the exposure tolerance.

[edit] Reading DX codes

Speed

ISO speed	1st row DX contacts
25
32
40
50
64
80
100
125
160
200
250
320
400
500
640
800
1000
1250
1600
2000
2500
3200
4000
5000

Film length

Exposures	2nd row DX contacts
other
12
20
24
36
48
60
72

Exposure tolerance

Exposure tolerance (in f-stops)	2nd row DX contacts
±½
±1
+2 -1
+3 -1

[edit] See also

The full DX geometrical and electrical standard is set by ANSI and I3A.

• DX number