Usefulness/Uselessness of DNG as an archival format

[freixas]

With Photoshop CS6 no longer getting RAW updates and me looking at some new cameras, I've been considering alternatives.

One alternative is to convert my (future) CR2 files to DNG—I could then continue to use Photoshop C6. So I've started reading about the pros and cons of DNG files.

A number of places mention that adjustments to the RAW data are included in the DNG rather than to a sidecar file. The benefit is that you can deliver a single file with all adjustments rather than the file plus the XMP sidecar. The drawback is that this then becomes a problem for backups since the entire large image has to be backed up, rather than the tiny sidecar file.

I've also looked at Photoshop replacements, including ON1, Affinity, and Capture One, as well as Adobe Camera RAW replacements like Rawtherapee. In testing all these programs with RAW files, I found that they all have their own version of a sidecar file. The usual explanation for these is that RAW adjustments are program-specific and so don't easily transfer over. (Note: ON1 2019 can actually convert Adobe adjustments for their own software, but only for Lightroom users. The conversion is not complete and not straight-forward).

It occurred to me that if the adjustments are program-specific, then even though they are included in the DNG, they provide no archival benefit. I was thinking of a DNG as an archival format for the image I had created. It's not. It's an archive format for the original RAW data, which can also get cluttered with vendor-specific adjustments.

XMP files seem to be an Adobe-only thing. When they are written to the DNG file, they make the file bigger, but neither help future-proof my adjustments or allow me to transfer the adjustments from one program to another.

I tested this by going to Adobe Camera Raw and making some changes to a DNG file. I then opened the DNG file using ON1, Rawtherapee, and Capture One. None of these saw any of the adjustments. In theory, ON1 could have converted the adjustments, but chose not to do so. Adobe, of course, ignores everyone else's adjustments.

In any case, I mention all this because I have not seen it stated in any of the DNG pros and cons articles I've been reading. If you're interested in archiving your final image, some other format would be a better choice. Unfortunately, I don't know of any non-destructive archival format.

By the way, the ON1 conversion is interesting. It's easy to read the Adobe settings (the XMP file is just a text file—you can read it with any text editor). But the meaning of the values is not just different from what ON1 uses, but also changes depending on the RAW file. To understand what a setting does, you would have to know Adobe's algorithm, something they are unlikely to share. ON1 had to create a special reverse-engineering algorithm to figure out how to do the conversion.

 

[chuasam]

On my D810, the NEF files were 70MB a piece. The DNG version of the same image was 35MB so I converted everything to DNG to save space.
When I upgraded to a D850, the NEF and DNG were both about 50MB a piece.