DX Coding
The pattern of conductive and insulating squares on a 35 mm cassette that tells the camera the film's speed and length.
Measured in
ISO
In depth
Contacts inside the film chamber read the pattern and set the ISO automatically, which removed a common and costly error. Photographers who wish to rate a film differently must either override the setting or modify the code with tape — a routine trick when pushing or pulling a roll.
Examples in practice
Covering part of a 400-speed cassette's DX pattern with tape makes the camera read it as 200 for a deliberate one-stop pull.