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 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.
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 film tolerance, or latitude, in ±f-stops.
Or graphically (with spool post to the left):
| G | S1 | S2 | S3 | S4 | S5 |
| G | L1 | L2 | L3 | T1 | T1 |
Where 'G' are the two common-ground contacts, 'S' is the film speed, 'L' the film length, and 'T' the exposure tolerance.
Reading DX codes
| 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 | ||||||
| Exposures | 2nd row DX contacts | |||||
|---|---|---|---|---|---|---|
| other | ||||||
| 12 | ||||||
| 20 | ||||||
| 24 | ||||||
| 36 | ||||||
| 48 | ||||||
| 60 | ||||||
| 72 | ||||||
| Exposure range (in f-stops) | 2nd row DX contacts | |||||
|---|---|---|---|---|---|---|
| ±½ | ||||||
| ±1 | ||||||
| +2 -1 | ||||||
| +3 -1 | ||||||
See also
The full DX geometrical and electrical standard is set by ANSI and I3A.
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