DX coding is probably the most understated piece of technology to be applied to film. Originally invented by Kodak, it is now used by all film manufacturers that I know of. DX coding is a three part system. What we mostly think about when talking about DX coding is the bare metal patches on film cartridges, but there is more than meets the eye.
The first part of the system is the barcode next to the 35mm cartridge's lip. This code identifies the manufacturer, film type and required process. It is mostly used by photofinishing machines to correctly process the film.
The second part of the system is the on-film barcode. On the edge of the film is printed a barcode. It consists of two different codes stacked on top of each other. The first one is the clock synchronizing code which is used for calculating the image position on the film. The second one contains film type, manufacturer and frame number.
The third part is certainly the most interesting part for us. It is the metal contacts on the exterior of the can. These patches relay to the camera a ton of information about the film. The first camera to use this information was strangely not by Kodak but by Konica! In 1985, the Konica TC-X was introduced and is said to be the very first camera to use DX coding. But getting back to the DX coding, the system is based on a patch of 12 contacts in a grid pattern. The system is designed to transmit to the camera all possible sensitivities from 25 ISO to 5000 ISO. On all cartridges, patches 1 and 7 are always open. These indicate to the camera that the inserted film cartridge is DX coded. Contacts 2 to 6 (right hand column) are used to indicate sensitivity from 25 to 5000 ISO. But sensitivity is only part of the information that is encoded. Very few cameras use the contacts on the left hand column. Patches 8 to 10 actually indicate the number of frames on the roll of film. There is the regular number of frames that can be detected like 12, 20, 24 and 36 exposures. There are also some very rare numbers like 48, 60 or 72 exposures that can be encoded. I must say that I have never seen film with that many possible exposures! Last but not least are patches 11 and 12. These indicate film latitude in EV values and it seems these were never used by any camera ever made. This is sad because it could have made exposure meters even more precise.
Now, let?s take a look at the more interesting codes. Exposure latitude is probably the most important things for photographers. Even though camera meters don?t take this into account, knowing the ?numbers? can be very helpful. Instead of going through the manufacturer?s recommendations, just looking at the film cartridge before popping it into the camera can be somewhat simpler.
Sensitivity codes are used by all modern cameras. It should be noted that if your camera has less than 6 contacts, it will not be able to detect intermediate sensitivities. By looking at those contacts, you will be able to figure out which films can be used. Often, the technical data will say that a camera can use films from 100 to 1600 ISO but will never say that intermediate sensitivities can?t be detected. Compact cameras will almost only look only for patch number 4. This means that the camera will default all lower sensitivity film to less than 320 ISO (usually 100ISO) and expose all film with more than 400 ISO at this same value.
Now I can see all the hackers among us find ways to code those un-coded film cartridges.
Hope you enjoyed it.
