03 March, 2014

DNG, Pt 1

Even if you're a Lightroom or Adobe Camera Raw (ACR) user, you don't need to use DNG. It's entirely optional. So, I'm going to tell you why I use it—after I tell you what it is.

Explaining DNG seemed pretty straightforward before I started writing. It became apparent, however, that considerable context is required. So, I'll do this in two parts. In this first part I'll explain what RAW and DNG are (as opposed to JPEGs), and then explain the benefits that led me to use DNG. In the second part I'll get into some of the implications and problems.


When you set your camera to record an image as a JPEG, the camera evaluates the incoming image and adjusts it to make it look like what the camera's designers thought that the photographer expected. More goes on than I'll describe here, but I'll try to hit the high points: In making a JPEG, the camera corrects for chromatic aberration and distortion; it sharpens the image, adjusts the exposure and contrast, guesses at the white balance and may do some noise reduction. It squeezes the image into a colour space (usually sRGB), reduces the image into an 8-bit format (256 levels in each of the three colour channels of red, green and blue). And, finally, it compresses the image to make it even smaller by deleting information in a manner designed not to affect the apparent quality of the image any more than necessary. (Because information is lost in that process, it's called "lossy compression.") Cameras usually do a damn good job, and you can use the resulting JPEG immediately, and just about anywhere.

RAW images, on the other hand, skip that in-camera processing. The unadjusted image is simply written to your memory card, usually in the camera company's proprietary format. (It's not as simple as this, however, because many companies can't resist some RAW fiddling, but for the sake of this discussion we won't go into that.) With RAW files the image elements mentioned in the JPEG creation process will be decided in post processing using image manipulation software. Enter, (ta da) Lightroom and ACR. 

When you import your RAW files into Lightroom or ACR you're faced with the option of either keeping the files in the proprietary RAW format of your camera, or converting them into DNGs. 

DNG is an image format created by Adobe as a standard to deal with the alphabet soup of proprietary formats. And, in addition, it's designed to add capabilities that the other formats lack—we'll come to those. The idea is to have a format that can be used as a digital negative. 

Many users don't convert, but I do—here are my reasons:
  • I want to "futureproof" my images.
  • Once an image is a DNG, there's no need for external, associated ("sidecar") files, everything remains in the one DNG "container."
  • There's a small saving in file space.
  • I'm confident that I'm not losing any quality in the translation.
  • I can find, view, and use DNGs in other programs.
  • Adobe has a plug-in for correcting colour casts that only works with DNG files
Future Proofing

DNGs will never be as popular or as ubiquitous as JPEGs, but they're already pretty popular; if only because DNG doesn't have any real competition. And DNG is an open standard. DNGs are used, for example, as an archival format in a project funded by the Library of Congress. And, as Lightroom and ACR become more popular, so does DNG.

One file

In Lightroom, if you choose to import your original RAW file in its proprietary format, any Lightroom actions saved to metadata are held within an Extensible Metadata Platform (XMP) "sidecar" file. That separate file must remain in the same folder as the original file and keep its associated file name. With DNG, all saved metadata is held within the one file. You can even tell Lightroom to embed the original file into the DNG file for possible extraction later. (That does increase the files sizes substantially.)

Smaller files

There a small, but real, size advantage in using DNGs—with no loss of quality. You don't see that every day.

Confidence in its quality

DNG is built upon the tagged image file format for electronic photography (TIFF/EP). TIFF, like DNG is an open standard (i.e. not proprietary). The default DNG is a "lossless" format, so it contains all the usable data from the original file.

Accessible through other programs

In the event that a DNG file is disassociated from Lightroom, it's still accessible. I can see my DNG images using Windows Explorer, for example; or through other programs such as Picasa. And, if I double-click on a DNG, it opens directly into Photoshop.

Using Adobe's "DNG Flat Field" plug in

Colour casts can be a problem at the widest end of the Sony 10-18mm zoom, for example, and other super-wide lenses on the the NEX-7. These casts can also arise as unwanted effects in tilt/shift work with technical cameras or with specialised tilt/shift lenses. Without the plug-in from Adobe Labs, such colour casts are extremely difficult to fix. There's also a separate program called "Cornerfix" that works like the "DNG Flat Field" plug in. It too, however, requires the image to be in DNG format.

DNGs work for me, but I'm not blind to the difficulties. I just haven't mentioned them here. For that you'll have to wait for part 2.

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