Sure thing. I will break them down them according to three categories.
Licensing issues
These come straight from the DNG Specification Patent License.
- “Adobe may revoke the rights granted above to any individual or organizational licensee in the event that such licensee or its affiliates brings any patent action against Adobe or its affiliates related to the reading or writing of files that comply with the DNG Specification.”
- “Subject to the terms below and solely to permit the reading and writing of image files that comply with the DNG Specification”
- “Adobe provides the DNG Specification to the public for the purpose of encouraging implementation of this file format in a compliant manner.”
- “All rights not expressly granted herein are reserved.”
These may all be “fair” in terms of a general copyright license, but they are things that would not be in a truly libre software license. In fact, according to the patent license linked above, I’m not at all sure that DNG can be included in a truly libre project (although IANA patent lawyer). Adobe remains in express and complete control of the specification, and so is the one driving the ship here, not the community.
The DNG Specification
All of these come from the current version of the DNG Specification you can download from Adobe’s website.
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You can use TIFF-EP, EXIF, IPTC, or XMP tags for metadata, but only in the standard tags, or the extra tags that Adobe has added. If you want other metadata, this is what they say: “Camera manufacturers may want to include proprietary data in a raw file for use by their own raw converter. DNG allows proprietary data to be stored using private tags, private IFDs, and/or a private MakerNote.” To me, this means “we will allow you to pigeonhole your non-adobe approved metadata into our container, but we will never make any moves to help you make it easy, or to ever bring those fields into the standard tag set, so if you want these metadata to be read, you have to make a special extra reader to get them in and out. Good luck with that!”
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“Lossy JPEG (34892) is allowed for IFDs that use PhotometricInterpretation = 34892
(LinearRaw) and 8-bit integer data.” So, it is possible to have lossy compression inside DNG in some circumstances. The end user of a program may not know that, and accidentally use lossy compression when they wanted a true “archival” format. This may not commonly happen, but IMO, it would still be better for a camera raw format to never use lossy compression.
General Issues
Adobe is now on version 1.4.0.0 of their DNG specification. Looking at the changes between versions, they are almost all of them to do with adding tags for new features/capabilites in Adobe Lightroom and or Photoshop. Therefore, it is pretty clear to me that, since Adobe maintains control of the specification, they mainly want it to be “open” to the public so as to entice people into using their own software. Indeed, you can only get the most out of the DNG format if you set up your software to record or need the same types of information that Adobe products take. This stifles creativity and freedom.
Case in point what I said above about Adobe Raw Converter. In fact it is in the very same RawPedia article you linked to that I paraphrased from the following quote: “Adobe’s DNG Converter is not the only program that converts raw files to DNG … however it is unknown what matrices this converter uses or where it gets them from, therefore it is safer if you stick to using the official Adobe DNG Converter.” (Emphasis added by me). Since this only runs via WINE, and it is not libre, it not a great solution for Linux users. And, if this truly is the safest way to convert DNG, then it is really a bad solution for Linux users.
Not all companies are bad, and there are reasons to limit data storage formats to certain specifications. But, do we really want a company who’s main business is selling (very expensive) software to be the ones in charge of creating the specifications??