DNG - yes or no?

Started Jun 8, 2009 | Discussions thread
BarryWheeler Junior Member • Posts: 40
Re: DNG - yes or no?

Please note, these are my own personal observations and do not reflect official policy of the Library of Congress.

The Library of Congress is concerned about storing digital files over a very long period of time. Currently we use TIFF for our master files. We may also retain a "production master" file which we process and convert into the images delivered through our various websites. These production master files are also generally TIFF; JPEG 2000 files are used for production masters for large files such as newspaper and map images but even for these files we retain the original TIFFs. You can find the Library's evaluation of the long term viability of many different formats at our website: http://www.digitalpreservation.gov/formats/fdd/still_fdd.shtml

We are beginning to consider the various RAW formats and DNG for two reasons. First, processing techniques developed in the future may be able to improve the output quality of our images. RAW and DNG files are the most likely to benefit from new postprocessing techniques, particularly since TIFF files cannot be edited without changing the actual image pixels. Secondly, we use many different cameras and scanners from many different manufacturers. We aslo tend to accept any files donated to us and photographers may start to provide RAW or DNG files.

While NO DECISION HAS BEEN REACHED by the Library, two factors support DNG as a preferred format. First, the multitude of RAW formats are difficult to manage, especially over time. (Personally I have already owned 4 digital cameras that produced RAW formats and have worked with 2 additional cameras - over the next 10 years I can expect to acquire and use many more cameras - each with unique RAW formats!) So think of the problem the management of RAW formats from dozens of different people and cameras! If you need to deliver or exchange image files, the variety of RAW formats quickly becomes a problem.

A second problem in the exchange and long term storage of RAW formats is the limited metadata that can be embedded in the file. The Exif format includes extensive technical metadata, but provides support for only a subset of the needed descriptive and copyright metadata. XMP can be embedded into DNG, providing much more robust metadata sets.

However Adobe has not yet convinced many people of the accuracy/adequacy/stability of the RAW to DNG conversion process. There are also questions about the long-term viability of DNG. I cannot speak for the Library, but for myself, I'm saving everything in RAW but preparing to move to DNG when and if it becomes an ISO standard.

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