Rudy Winston
Rudy Winston

Rudy Winston has over 17 years experience with Canon USA's Pro Products team, and has been responsible during that time for training Canon's staff on new products, creating presentations for customers and dealers, numerous writing projects, and providing technical assistance to professional and amateur photographers.

Why do digital cameras produce odd file names?

July 08, 2014

If you’ve sat awake at night, wondering why your shiny new EOS camera seems locked-in to the same strange litany of file names that your old 1.0 megapixel digital point-and-shoot from 2002 produced, you’re not alone. It’s not a lot of help to know that your picture of that historical landmark is file number “IMG_4238.JPG,” and if you’ve wished your camera could re-name it “Eiffel Tower at night, Paris, 2014,” you’re not alone. But there are reasons why camera makers use this seemingly arcane numbering system, and it dates back to the early days of digital imaging.


The DCF standard, and a basic look at why we have it

This is strictly extra-credit reading, and it’s certainly not something the digital SLR user needs to know or understand to work with their cameras. The arcane-sounding names that Canon and other camera makers assign to the image folders you create on a memory card and, for that matter, the image files themselves, relates to their adherence to the DCF (Design Rule for Camera Filing Systems) industry standard that exists in the digital photo industry today.

The DCF standard was one created in the early days by a grouping of representatives from different companies, creating some order from what they doubtless feared might become a 21st century digital version of the “Wild West.” With the need for makers of memory cards, cameras, card readers, computers, and imaging software to all be able to work with each other’s products, uniformity was called for. The reasoning is sound; it would be as limiting to have camera files from one camera company that could only be recognized by one computer operating system or one brand of card reader as it might be for an auto maker to require only one brand of gasoline to be compatible with their cars.

The DCF standard enables computers, card readers, cameras, and so on to recognize the contents of a memory card or disk as digital image files, and to be able to access and work with them in predictable ways — no matter what camera created the images, or what computer is being used to view them.

Like a lot of standards, it can sometimes seem a bit confusing and the fact that it originated in the early days of digital photography mean that some things, like 128GB memory cards or cameras that could shoot at 12 frames per second, probably weren’t considered when the standard originated.

The nuts and bolts of the DCF file system
File structure:

Uniformity starts with the way files are laid onto the card. The industry-standard DCF system calls for one master folder, called a DCIM folder (Digital Camera IMages) to house all created content. One DCIM folder must be present on your memory card and it’s created by the camera whenever you format a card in-camera.


Folders — “DCF Directories”

The sub-folder(s) that house your actual images are formally called DCF Directories, but that’s the last time we’ll mention that term here. The DCF system has a file naming structure for these image folders, for uniform recognition, industry-wide:

  • Name — 8 characters in length
  • First three characters — Directory Numbers 100 to 999 (000 to 099 are not allowed)
  • Next five characters — Free upper-case alphanumeric characters, or underscore "_"

Hence, the use of folder names like "100CANON" on your EOS digital SLR, as well as the reason the camera won’t let you change that folder name in-camera. By definition, the DCF system requires one folder to be created within your DCIM folder; many cameras — including most current Canon EOS models — allow you to create up to 899 total folders for images on the memory card.


Image File names:

These are the actual names/numbers assigned to each image file you take by the camera. File names also conform to the DCF structure, in terms of what the camera can assign in-camera.

  • Name — 8 characters in length
  • First four characters — DCF file name Free Characters: upper-case alphanumeric, and/or "_"; lower-case letters are treated as upper-case
  • Four following characters — File Number, 0001 thru 9999, individual for each file in a folder (duplicate numbers not allowed in same folder)

Again, here is why your camera delivers images with file names like “IMG_2345.JPG.” While it’s true that high-end EOS cameras, like the EOS-1D series, allow users to change the first four characters of file names freely, they still conform to the above-mentioned “free characters” protocol and require four following digits.

The bottom line:

It would often be cool if we had total freedom to change file-naming in-camera, but there are reasons — debatable, for sure — why our camera industry doesn’t let us do this at the time a shot is taken. It’s true that many types of browsing software allow us to re-name image files as we download them to the computer or after they’ve been saved to a destination folder on a connected hard drive. None of this will prevent those occasional sleepless nights some of us experience, fretting about the complexities our digital cameras present to us. But the purpose here is merely to shed a little light on how this numbering system came to be, what its definitions and ranges are, and to understand what you can and can’t do within this system.

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