Here's what UPDIG has to say about Digital Archiving and the various trade-offs between RAW, DNG and Tif.
I find no major fault with this overview analysis. UPDIG is a work-in-progress and a very good head start as a basic practice model for museum imaging practicioners.
Alan Newman
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Archival image formats RAW file archiving for DSLR users is likely to be one of the greatest challenges in the long-term maintenance of a digital archive. Because each camera model creates a unique file-type, the likelihood of files becoming unreadable at some time in the future is high. Photographers must develop a comprehensive strategy to use when confronting this issue: one that takes into account the probable obsolescence of file formats and the necessity of file format migration. File format migration refers to the practice of conversion of image files to a different storage format. This can come in the form of conversion to TIFF files, conversion to DNG files, or conversion to some future format not even in existence today. Each of these choices holds certain advantages and disadvantages regarding image quality, storage needs, and workflow requirements. Some of these concerns are outlined below. Conversion to TIFF files By converting images to TIFF format, the photographer is storing the images in the most accessible file format. Because TIFF is an open standard, it is likely to be readable for a very long time. TIFF also offers a workflow advantage: By converting to TIFF, you probably eliminate the need to reconvert the files again for many years, perhaps even for your lifetime. So images can be converted and archived with confidence that they are safely accessible long into the future. There is a downside, however. TIFF files are much larger than RAW files. Converting image files to 16-bit TIFFs can make the files up to 10 times larger than RAW files, and 15 times larger than DNG. This will clearly increase the cost of file storage. The other downside to conversion to TIFF is that it precludes the use of better RAW converters that are surely coming in the future. Just as Photoshop CS2 does a better conversion that Photoshop CS does, it is likely that the RAW file conversion programs available in several years will do an even better job than our current tools. Archiving RAW files If a photographer chooses to archive the RAW file, then he will be preserving the largest number of options for future conversion of the files. As conversion software improves, new versions of the file can be made that will can have better color fidelity, or better noise reduction, or better upsizing interpolation. By keeping the RAW file intact, the largest number of future conversion options is preserved. This, too, has its downside. RAW files will likely have to be converted to a more universal file format at some time in the future. This may involve the conversion and re-cataloging of hundreds of thousands of image files. If this conversion is not accomplished before that particular format becomes generally unreadable, then the conversion may simply never happen, and those images may be lost. Additionally, since RAW files are proprietary, it is not safe practice to alter these files. This means that any work you do to these files, such as the addition of metadata, or adjustments to the image, cannot be stored in the file itself. This is typically accomplished by the use of either “sidecar” files, or the storage of these adjustments in some kind of larger database. The splitting of this information makes for a file-management challenge that may present a serious roadblock in the future, as you try to include this work in a conversion file. Archiving DNG files RAW files can be converted to DNG files, which is an open format that can store the RAW image data, metadata, and a color-corrected JPEG preview of the image. The DNG file format provides a common platform for information about the file and adjustments to the image. Because of this, cataloging applications like iVIew MediaPro and Extensis Portfolio can see metadata that has been entered in Photoshop, and these programs can see the image adjustments made in Photoshop. DNG files can be re-opened in Photoshop as though they were RAW files, and offer the full range of conversion options in Photoshop that the original RAW file offered. DNG is likely to be readable long after the original RAW format becomes obsolete, simply because there will be so many more of them than any particular RAW file format. Additionally, the DNG offers significant file-size savings through a lossless compression that can reduce the file size by up to one third. Also, by converting to DNG at the time of archiving, you are likely to forestall further file format migration for a very long time. DNG also offers the possibility of embedding the RAW file itself, so that it can be re-converted later if desired. Of course, this particular option makes the DNG almost double the size of the original RAW file. There’a a downside to DNG, of course. Conversion to DNG requires an extra step at the time of RAW file processing; it does not take terribly long, but it is an extra process. Also, by converting a RAW file to DNG, you preclude the manufacturer’s software for converting this file. If you like the conversions you get from the manufacturer’s software more than the conversions you get from Photoshop, then DNG may be unacceptable. The DNG converter attempts to copy “Undocumented Maker Notes” to the metadata of the DNG file. These maker notes might include information that could be useful in converting the file at a later date (such as “dust reference” information, or information helpful in correcting for chromatic aberration). By converting to DNG today, you may lose this data, since it is currently undocumented and it may not be copied correctly.
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