Getting DNGed

A commonly asked question. A not-so-common answer.

Every now and then I get a batch of queries about the DNG format, usually when articles appear elsewhere about why you should convert to DNG. As many of you probably know, I like the idea of an open and well defined raw format, but I'm not particularly supportive of using DNG, for reasons I'll get to in a moment.

The usual benefits that are ascribed to DNG are:

  • It's an open definition, so it'll be around forever.
  • Many software products support DNG naturally; you don't need a new raw converter update when a new DNG-capable camera comes out.
  • Your files will get smaller due to the way DNG stores things, at least if you let the DNG converter throw away the original data file instead of retaining it as a backup.
  • DNG relieves you from the tyranny of the manufacturer's proprietary formats (okay, that's a bit overstated, but I've had DNG supporters actually say this to me).

DNG was proposed and documented by Adobe back in 2004, and Adobe supplies a free DNG converter that will take your raw files (e.g. NEFs) and spit out DNG files. Adobe has asserted that they'll hand the DNG specification over to a standards board (and they submitted it to the TIFF/EP standards committee), but standardization has not happened yet and thus DNG is not a standard, as some have implied (or even stated). It's just another raw format, though it's been fully documented by its creator. A few camera companies have adopted DNG as their raw format, but not Nikon or Canon, the two largest DSLR players. 

When people ask me if they should convert their NEFs to DNG, my usual answer is "no." The reason is simple: the potential benefits seem to be greatly overshadowed by the drawbacks right now. That may change someday, but the bottom line is that the drawbacks currently outweight benefits in my opinion. Consider:

  • If you're shooting with a non-DNG camera (such as a NEF-creating Nikon DSLR), you increase your workflow steps and import time by adding another step: the DNG conversion.
  • Some Nikon compressed NEFs will grow in size when converted, not decrease in size as claimed.
  • The black point is set during the conversion to DNG (usually it's only set in the final conversion). Masked black pixels aren't carried over, either, which can be used in some raw converters.
  • The DNG converter doesn't always contain the embedded JPEG from the raw file (this is optional). This is important because:
  • No raw converters that interpret the DNG file will match what the Nikon camera's do in-camera (embedded JPEG). By converting to DNG you're essentially saying that you'll forgo the camera maker's settings, styles, and color rendering. (That's a big issue, in my mind, as the camera makers don't document the spectral response of the Bayer filtration.)
  • If you convert to DNG you have problems with some raw converters down the workflow line: DxO can't do its magic with a Nikon NEF converted to DNG last time I checked. And Nikon's own free converter, NX Studio does not accept DNG, either. 
  • If you elected to keep the original raw file with the DNG (recommended), you've escalated your storage and backup requirements.
  • Sidecar modification files should be kept with the original DNG. This means you need to watch your backup systems and make sure they keep file and folder structures intact.

Again, I'm not against the idea of open, documented raw file formats. I'm actually very much for it. But unless Nikon, Canon, and others decide to support DNG and start documenting the inner specifications of their systems, we're in a "manufacturer-knows-best" versus "we-can-reverse-engineer-it (kind of)" type of fight, with the maker's raw formats on one side and DNG on the other. The downstream benefits of converting to DNG are just not dramatic enough to move savvy users into the DNG camp, in my opinion.

Oh, and there's this thing some DNG proponents keep trying to scare people with: "some day your camera maker might no longer support your raw format." So what? We already have open source code that interprets NEF (see, amongst others). If the future some day will require you to move to another file format, you can do so then. In the meantime, for DNG to succeed the short-term benefits need to be bigger and clear. I don't believe they are.

Let the arrow-slinging begin... 

All that said, there is one particular use of DNG that might be of interest to some: when the software you like gets updated so that it no longer runs on your system (or you want to get off the software upgrade bandwagon). For example, Adobe is no longer supporting older machines and OS versions with some of their recent updates (e.g. Lightroom 4.x doesn't support Windows XT; but if you buy a new camera, the only way you can get conversion support is to update from Lightroom 3.x to Lightroom 4.x, which means you have to upgrade both software and OS). You can download the free DNG converter from Adobe and use your old converter (which should understand DNG) to avoid having to upgrade costly hardware/software. Personally, I'm not a fan of this approach. If you're in a tech world, you really need to move all your tech upwards close to simultaneously over time or else you start getting additions to workflow (e.g. conversion to DNG) and/or idiosyncrasies (e.g. not sure how well the X-Pro1's non-Bayer sensor is going to be supported by older DNG converters).  

Finally, running a raw image through the DNG Converter may allow you to take out the automatic lens corrections that a lot of the makers now insist on. It's really the only reason I ever use DNG.

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