Film DX Coding – A photographer’s life hack
Film Digital Index (DX) Coding can be very convenient for 35mm. No need to worry about setting the film speed (ASA, ISO etc.) on the camera settings to ensure it meters correctly, just insert film and shoot away. This function is especially prevalent on 80s and 90s compact film cameras, some of which are very popular and getting expensive now (looking at you Olympus mju Stylus).
The problems start to arise when the box speed of the film is not what you require or want. For instance, you may want to shoot Ilford HP5+ but it is a bit of a gloomy day and you would like to push the film one stop, from ISO 400 to ISO 800, to increase contrast and allow more sensitivity to light. Alternatively, you may want to shoot Kodak Portra 400 at ISO 400 but you rate Portra at ISO 250 in camera, specifically to get the beautiful saturation of colours it can produce.
One other problem you may face is when the film cartridge may not have DX coding. This is a situation with some “boutique” films available or even when you load your own film from bulk rolls. A lot of compacts will default to ISO 100. Some do get around this, like the Olympus XA3, where you can only manually set the ISO if the DX code is not there or is hidden.
If you load film into a SLR which allows you to override the film speed by adjusting the ISO setting, you have control over this, but in a compact camera this may not be possible. The camera reads the DX coding and sets itself up as ISO 400 in both the above cases of HP5+ and Portra 400, and that is that.
There are, though, several ways around this, some a little more brutal which involve scratching the film cartridge and others which allow you as the photographer full control but requires a little planning and preparation. Before we look at one specific option on how to overcome the DX Coding defaults, let’s find out a bit more about it.
In January 1983, Kodak introduced its Film DX Coding solution, one of the many innovations which it has left in the world of photography. It succeeded attempts by other manufacturers and was hailed as a good solution due to the fact that it covered film identification end to end, not just when shooting.
The system met success in the market place immediately and was adopted by many camera and film processing equipment manufacturers within a few years. The first film to incorporate DX coding was, unsurprisingly, Kodak’s own Kodacolor VR-1000 film within the same year. The first camera was actually not a compact but the Konica TC-X SLR. Other cameras from Pentax and Minolta followed closely.
The DX Coding system has been the standard in the photography industry since then and 35mm films today are still produced with this system.
Most people assume the DX Coding system only relates to the coding on 35mm cartridges. That is incorrect, as it is also incorporated into the film itself. This allows for photo printers to identify the film type even when film is returned for a re-print. It is an ANSI standard recognised globally.
Specifically, for the cartridge it is a grid with Camera Auto Sensing contacts attached to the cartridge itself. It is formatted in two rows, each with a series of six rectangular areas, which are either black or metal electrical contacts. It is these electrical contacts which the camera “reads”. Not all cameras use the second row.
The top row identifies the film ISO speed. Depending on the combination of these contacts, the top row represents speeds from ISO 25 through to ISO 5000. There are also eight custom combinations available.
The second row is split into two parts. The first four contact areas identify the number of exposures the cartridge contains in regular 135 format, from 12 exposures through to 72. There is also an option for “other”. The final two contact areas allow for identification of the film tolerance, with the options being half an f-stop over/under, 1 f-stop over/under, 2 f-stops over and 1 f-stop under and finally 3 f-stops over and 1 f-stop under.
The options can be clearly seen in the tables below (which you are welcome to click on and then save the picture for your own reference):
As DX Coding is an end to end solution, it allows for coding on the film itself. This is for printers to identify the film for printing once it has been developed. As this article is for shooting at different film speeds, it is only being mentioned for reference. The barcode looks like the example below, at the bottom of the frame.
Now let’s look at how to overcome the automatic DX Coding when loading film into a camera. You will need the following to do this:
- Avery or similar labels for an inkjet printer
- An inkjet printer
- Either a sharp craft knife or a hole punch
- Tin foil
- Double sided tape
- Single sided tape (optional but recommended)
To demonstrate this, I will use a Kodak Ektar cartridge, where the film snapped in a camera I was using, so is empty. It will avoid the film being wound on by the camera in the demonstration.
As you can see in the image below, Kodak Ektar is an ISO 100 rated film. We can see this from the DX Coding.
The DX Code first row identifies the film as ISO 100, while the second row tells us that the cartridge has 36 exposures and the film has a tolerance of ±1 f-stop of over/under exposure.
To test out the reading I inserted the cartridge into a Nikon F-801 set to DX mode on the film speed option. Below it shows that the camera recognises this film as ISO 100.
The next step is to create the new DX Code for the cartridge. This is done by affixing a label to the cartridge which covers the one already provided by the manufacturer. I use Avery labels for this and Microsoft Word to design and print the new codes. Avery provides templates for their labels online free of charge in Word format. I have measured the size of the DX Code and created a small table inside of the label boundaries, as per below.
In this example I have filled in the top row with black using the codes to specify an ISO 50. I have replicated the ones on the second row to leave as 36 exposures and the manufacturer’s tolerance. I have also added a little reminder note at the bottom of what this is set to. As my labels can fit two, I always create minimum of two each time.
The label is then printed out.
At this point it is still stuck to the backing paper which is where I will leave it while I cut out the contact areas. To do this I use either a knife or a hole punch, whichever is more convenient at the time. Make sure if you use the knife to have a cutting board behind the label.
After the contact areas have been cut out, the blank spots require something metallic for the sensors to connect with. Tin foil works well for this, I tend to use the less reflective side which is very similar to the contacts on the original film cartridge. After affixing the label to the foil, cut around the foil to make it the same size as the label.
Now that the label is ready, it is time to attach it to the cartridge. I do this with double sided tape.
I align it to where the contact slots are to ensure it will be readable. I also add a couple of pieces of single sided tape on the edges to make sure it does not accidentally come off. Scotch Magic tape works well.
As the cartridge is now ready, to test it out for the purposes of this demonstration, I insert it into the F-801 as per normal.
I am now at the big moment, does it work? As per the first test, I set the camera on DX and check the ISO.
As you can now see the camera recognises the ISO as 50.
I use this quite a bit in some of my cameras especially as I prefer to overexpose Portra 400 by just under an f-stop and consistently push HP5+ by 1, 2 and sometimes 3 f-stops. So next time you have this wonderful camera and want a little more control over the film speed, prepare a few sheets of labels and have them at the ready.
For some convenience, I have included for you, a downloadable copy of my Word template which has the contact area already sized up for printing. It is for the labels I use, but it can be copied across very easily. Just click on this link: Film DX Labels Template.doc
This is great! Thanks so much for including the template.
You’re very welcome!