In the last three years a mysterious group of checkerboard-style squares have found their way onto the cylindrical cartridges of 35-millimeter film. The silver and black patterns are not merely decorative; they are the most obvious manifestations of a new method of automatic film identification known as the DX coding system.

The DX code was developed by Kodak to meet the challenges of increasingly electronic camera design, automated film processing, and the demands of a public unwilling to deal with putatively complicated camera settings. Why anyone should feel that turning an aperture or shutter-speed ring is beyond them escapes me, but there's no doubt that a fail-safe system for setting ISO/ASA film speeds is a boon to care-free photography.

This is just what the DX system does: it automatically signals to the camera, by means of those checkerboard squares, the sensitivity of the film in use. It can also tell the camera how long a roll of film it is (36 or 24 exposures) and how particular the film is about receiving the exactly correct exposure (what's called, in photo jargon, the exposure latitude of the film).

The DX system can only function, of course, if the camera is intelligent enough to recognize its messages. Many of the newest models, particularly point-and-shoot 35-millimeter types, do this. Open up the back of any of these cameras and examine the chamber that holds the film cartridge; if a row of gold-plated metal contacts is visible, the camera is equipped to make use of the cartridge's coded information.

Older cameras, and ones designed for more sophisticated photographers, don't have the contacts the DX system needs. Does this mean they are outdated? Not by a long shot. It simply means that the photographer remains responsible for setting the film speed and knowing the length and exposure latitude of the film.

Part of the DX system does function with all cameras, contacts or not. But this capability has to do with processing, not picture taking. Besides providing cameras with information, the system can also tell automated processing labs just what kind of film is inside the cartridge and, if it is negative film, how to print it.

These features are accomplished not with the silver-and-black checkboard markings but with the bar codes that appear both on the cartridge and on the edges of the processed film. Resembling checkout-counter hash marks found on cereal boxes, which tell the cash register how much you owe, these bar codes reveal their secrets to laboratory processing and printing machines equipped to interpret them. Kodak's processing plants, of course, are already fully integrated with the system.

Deciphering Kodak's DX code is as much of a task as solving Rubik's Cube, and about as useful. You don't have to know how it works for it to do its job. But for those with an unquenchable thirst to know, here are the cryptographic basics in a nutshell:

The bare or silver squares conduct electrical current; the black ones, which are painted, do not. Thus, a suitably designed camera can sense, via its gold contacts, which are which. The squares come in two rows of six. The first row, closest to the bar code and the felt lip of the cartridge, contains the code for film speeds. The second row, nearest the name of the film, is devoted to film length and exposure latitude.

The two squares that are on top when the film is sitting flat on its base are always bare. That leaves five squares for film speed, on the right. Three of these cover broad, full-stop differences in film sensitivity, while the bottom two refine things by one-third stop increments. If all three squares under the top bare square are black, the camera reads the film as ISO/ASA 25. If all three are bare, it means ISO/ASA 3,200. All other speeds are handled by various combinations in between. (For math puzzle wizards, and others who want to figure it all out precisely, here's a clue: Kodachrome 64's DX code for film speed reads silver, black, black, followed by a black and a white square in the fine-tuning zone.) The second row is somewhat more straightforward. The three squares after the top one are capable of indicating exposure lengths from 12 to 72. Since Kodak currently restricts itself to 24 and 36, only one square - the fourth from the top - suffices. In computerese, it's a toggle; on is 36 exposures, off is 24.

If the last two squares are black, the latitude of the film is a half stop in either direction, meaning slide film. If the same two are silver, the indicated latitude is greater, meaning print film. Thank goodness we can load DX-coded film like we load any other, and forget it.

As mentioned earlier, it doesn't matter whether a camera is equipped to use the DX code; the film will still perform as advertised. Even among cameras designed for the system there is some variance. Some have as many as 10 contacts, while others take a bare-bones approach with three.

The contrary is not necessarily the case. That is, a non-DX roll of film in a DX-designed camera may play havoc on the exposure system, giving film-speed readings that are way off base. So if you have gotten a new camera in the relatively recent past, peek inside the next time you take the film out and see if it has DX contacts. If it does, you should stick to DX-coded films. Fortunately, the DX system has been adopted by nearly all major film manufacturers, so you don't need to abandon your favorite brand.