The DX code is a black and silver block-pattern code on 35mm film cassettes, introduced in 1983. It is used by some cameras to set the ISO and by film processing machines to tell them about the film. All well and good, but sometimes we want to push or pull the film and this means setting a different ISO in the camera. If the camera can’t be controlled or over-ridden, you can change the code on the film cassette itself.
This is a typical code:
The code is read as shown – with the barcode and film at the top. The ISO coding is the top half of the code and forms six panels read left to right. The first panel is always silver/ metal.
And an aside – the lower half of the code panel shows the length of the film and its tolerance for over and under exposure. This one is 24 exposures and +3 to -1 stops.
You change the ISO by scraping the black paint off one or more squares or by covering them with tape. The camera uses electrical contacts to read whether a square is conductive (silver) or not (black).
There is a table below of the codes that correspond to each ISO.
So one of the common hacks would be to rate a 400 ISO film as 800. To do this you need to scrape the black paint off panel 2. To push it to 1600, leave panel 2 and scrape off panel 3.
For more information, plus how to decode the lower section, see here.
It’s also possible to make a completely new code by scraping all of the panels and covering some of them with tape. There has been a revival in using some of the other films in Kodak’s former catalogue. Many of these are known by code number instead of a common name so it may not be obvious what ISO to use. There is a handy decoding list here for Kodak and here for Fuji. So should you find yourself trying to shoot Kodak 2430 in an automated camera or trying to reload old film cassettes with a different film, help is at hand.
… And another aside – follow that link for an appreciation of just how many types of film Kodak made.