Monday, December 5, 2011

Using Digital Negative (DNG) to archive your raw image files

Five, 10, or even 40 years from now, will you be able to open one of the proprietary, raw image files from your digital camera? You will, if you have converted your raw camera files to DNG (.dng).

Why shoot in raw? Raw image files are sometimes called digital negatives, as they fulfill the same role as negatives in film photography: that is, the negative is not directly usable as an image, but has all of the information needed to create an image. Like a photographic negative, a raw digital image will usually have a wider dynamic range (16-bits per color channel) and color gamut than the eventual final image format (usually a JPEG, or .JPG, which is an 8-bit image). The purpose of raw image formats is to save, with minimum loss of information, data obtained from the camera's sensor. Here are three examples (one, two, three) of why you should be saving your camera's raw image format file -- if you aren't already.

Reading the raw file. Nearly every time I have purchased a new digital camera, I have had to purchase the latest version of Photoshop, or a new raw converter plug-in in order to read and open the new raw image files produced by that camera. Examples of proprietary raw image file format extensions are .CR2 (Canon), .NEF (Nikon), .ORF (Olympus), .PEF (Pentax), and .RW2 (Panasonic).

Free raw file converter. A year after I purchased my last DSLR, I could not open the raw camera files with my current version of Photoshop. I was not ready to upgrade to Photoshop CS5 (and pay another $200), because I had just done that the year before, when I upgraded from CS3 to CS4. A friend suggested that I get Adobe's free DNG Converter, and convert all my new camera raw files to the universal DNG format, which is supported by all the versions of Photoshop (back to the original CS), Elements (back to version 3), and Lightroom.

Reduced file size. Not only is the free Adobe Digital Negative Converter fast in its conversion, but it typically reduces the file size of my Canon ".CR2" files by 20% when they are converted over to the ".dng" format. I am now, systematically, taking all my old CR2 files, converting them to DNG, and throwing away the old CR2 versions.

Archival peace of mind. The best reason to convert your camera raw files to the universal DNG format is long-term compatibility. Because cameras can use many different raw formats -- the specifications for which are not publicly available -- this means that not every raw file can be read by a variety of software applications. As a result, the use of these proprietary raw files as a long-term archival solution carries risk.

DNG is an open raw image format owned by Adobe. DNG is based on the TIFF/EP standard format. Exploitation of the file format is royalty free. It was launched in 2004 after requests by many photographers and photo trade associations (i.e. the American Society of Media Photographers, or ASMP) that there be a universal raw format standard. The US Library of Congress recommends DNG as the preferred alternative to other "less desirable" raw image formats. In their "Digital Photography Best Practices and Workflow" (dpBestflow) project, ASMP (funded by the US Library of Congress) stated that "DNG files have proven to be significantly more useful than the proprietary raw files in our workflow." With this kind of support and recommendation, we should expect the DNG format to be around for a long, long time!

You are invited to see my "Most Interesting" images on Flickr (based on popularity stats).


  1. Thank you for the excellent and helpful information. I do appreciate it!


  2. ReaConverter was originally developed as a great time-saver for people whose work involves converting and editing lots of images every day. Many professional photographers, web designers from all over the world appreciate this program for its ability to batch convert images from 400+ source formats into 60 common and advanced output formats, like Convert CR2 to JPEG.

  3. Hi Royce,

    Recently, I attended one of your presentations and I believe you had ultimately decided that DNG wasn't a good method of archiving. Can you please elaborate on that for me (and others)?

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