DX is short for Digital indeX encoding. It is an ANSI and I3A standard for marking 135 and APS photographic film and film cartridges. It has many parts, a latent image DX film edge barcode on the film underneath the sprocket holes, a code on the cartridge utilized by automatic cameras, and a barcode on the cartridge read by photo-finishing machines.
Next to the film exit lip is an Interleaved 2 of 5 barcode and a printed number which are representative of the I3A assigned DX number, the total number of exposures and a proprietary manufacturer’s code. The DX number may be used to identify the manufacturer, film type, and by inference, the needed developing process type. This is employed by automatic photo-finishing machines to correctly and appropriately process the exposed film.
Below the sprockets beneath each frame of 135 film is the DX film edge barcode. The barcode is not visible until the film has actually been developed. It is optically imprinted as a latent image during the process of manufacturing. They are utilized by photo finishers to designate and align each frame for printing. It is composed of two parallel linear barcodes, one for a synchronizing clock, and the other for encoding film data such as type, manufacturer and frame number.
The outside of film cartridges are imprinted with a DX Camera Auto Sensing code which is readable by many cameras. Cameras will essentially be able to automatically determine the film speed, number of exposures and exposure tolerance. The first 35mm camera to implement the technology was the Konica TC-X, which was first put on the market in 1985.
The DX Camera Auto Sensing code manifests as a grid of contact points on the side of the metal cartridge surface that are either conductive or non-conductive. Electrical contacts within the body of the camera read the bit pattern. Most cameras will only read part of the code; usually, only the film speed is read, and some cameras marketed to the wider consumer market only read enough bits of code to differentiate the most common film speeds. For example 100, 200, 400, and 800 can be discovered by reading only S1 and S2 and ground.
On the 35mm film cartridges there are two rows of six rectangular areas. The two areas closest to the left which have the spool post on the left are both common and are thus always bare metal. The remaining 5 bits in the top row represent a total of 32 possible film speeds. But only the 24 speeds representing the 1/3 stops from ISO 25/15° to 5000/38° are actually implemented. The codes are not necessarily in strict binary order.
In the second row, the first 3 bits designate 8 possible film lengths, although technically in practice only 12, 20, 24 and 36 exposures are actually encoded. The other 2 bits of the second row give 4 ranges of exposure tolerance, or latitude.