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Tuesday, February 26, 2013

DNG File Format & DNG Converter

In this excerpt from his book, digital master photographer and Photoshop Alpha-tester Jeff Schewe demystifies the DNG format for today and for the future

This Article Features Photo Zoom

Given the very real problem of the rapid proliferation of proprietary and undocumented raw file formats, Adobe (read: Thomas Knoll) decided to do something about that. In September 2004, Adobe launched the initial Digital Negative (DNG) file format. The purpose of the DNG file format was to try to create some order out of chaos because the photographic industry has spawned way too many file formats with no real likelihood of a real standardized raw file format in the short term.

Figure 1: Rusty Truck image shot with a Phase One IQ 180 digital back
To address this issue, Thomas advocated a standardized raw file format that was more structured and publicly documented. DNG is compatible with the ISO 12234-2 spec for TIFF Electronic Photography (TIFF/EP), which is used by most camera makers in one form or another. Adobe has offered the DNG specification to the International Organization for Standardization (ISO) for an upcoming ISO update to TIFF-EP. Unfortunately, standards organizations are notoriously slow (and careful, which is a good thing) when it comes to revising existing standards. So, I can't tell what's going to happen about the TIFF-EP adoption of DNG.

Regardless of the political and technical implications of DNG as a file format (and it's mostly political, not technical), the one thing that can be said for DNG is that it provides a chance for backward compatibility for users with new cameras using old software. The free DNG Converter, which is released concurrent with new versions of Camera Raw, can convert newly supported raw files into a DNG file that can be read in older versions of Camera Raw and Lightroom.

Figure 2: Adobe DNG Converter
To DNG Or Not To DNG?
So, here's the thing: In the short term, I'm really not all that concerned about being able to access all my proprietary raw files in current versions of Camera Raw and Lightroom. I don't see Adobe ever removing support for any cameras whose raw files are currently supported.

In fact, there's a real benefit when working with Camera Raw or Lightroom to maintaining your original raw files without conversion to DNG. In Camera Raw and Lightroom, if you save your image settings into the DNG file, the modification date is updated. What that means is, if you use a backup application that backs up based on changed modification dates, a DNG that's updated will require backing up. To show you the implications of this, consider the image of the rusty, old truck (Fig. 1). The image was shot with a Phase One IQ 180 medium-format digital back. The back is an 80 MP back with a native file size of about 80 MB (give or take a few megabytes). Converted to DNG, the file size grows to about 105 MB (the DNG compression for IIQ files from Phase One isn't great). So, each time I adjust the DNG file image settings, my backup application will note the new file modification date and dutifully copy the 100+ MB file to backup.


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