Many times I’ve been asked by students and workshop attendees whether they should be converting their RAW photo files to Adobe’s DNG format. There are various pros and cons to this issue, so I’ll do my best to list them here along with where I’ve ended up on the issue. Hopefully it will help you review your own conclusions.
First, a quick intro: DNG (Digital NeGative) is a format devised by Adobe for storage of photographic RAW data. The standard is published by Adobe, and many software vendors have added support for it to their products (whether by using the libraries that Adobe provides, or their own software from the ground up – DNG is built on top of the well-known TIFF format). Rather than each camera vendor needing to have their own proprietary RAW format and make incompatible extensions to the format for each new camera model, DNG provides a common format. The standard does get modified over time to support new features but this is done in a backwards-compatible way. Some camera vendors have chosen to leverage this format and have their cameras produce it “out of the box”. Tools such as Adobe’s free DNG Converter, Camera Raw (a plug-in for Photoshop and Bridge), and Photoshop Lightroom can also produce DNG versions of any supported RAW file.
Incidentally, the TIFF standard was also devised by Adobe and published in the same way. Anyone concerned that DNG is an “Adobe-controlled standard” should have a look at how comfortable they are with the TIFF format that has become an industry standard. This isn’t an issue that concerns me.
So, should you keep the RAW photos from your cameras in their native format (e.g. CR2, NEF) or should you convert them to DNG?
- As of DNG 1.2, it supports an internal MD5 checksum of the RAW data within the file. Software can verify this to be sure that your files are undamaged. Note that currently there are no efficient tools that use this to verify large numbers of files (Lightroom 3‘s Develop module will just check the current image, the DNG Converter insists on creating new DNG copies with no verify-only option, etc). Hopefully better verification tools will appear soon.
- DNG files are usually smaller than the corresponding proprietary RAW files. Mind you, disk space keeps getting cheaper.
- The preview embedded into DNG files can be updated to represent the “current” processed form of the image. This means that if you’re using non-ACR software (i.e. something other than Bridge or Lightroom) to manage your files that the thumbnails and previews will not be stuck at the version of the image rendered by the in-camera JPEG processing. Note however that this preview updating is often not automatic (which would add a significant performance overhead).
- DNG provides a stable future-proof format for archiving of RAW images. If files are being committed to write-once media (e.g. DVD) this is a good choice. If files are being stored on hard drives then large volumes of proprietary RAW files can be accessed and updated as and if needed. However, current Adobe software can handle all the historical “official” RAW formats I’m aware of.
- DNG provides a RAW format for images from cameras that your current RAW-processing software doesn’t support. For example if you have a camera newer than that handled by your installed Photoshop/Lightroom version, you can either update your software or you can use the latest version of Adobe’s free DNG Converter and then process the DNGs.
Note that while DNG is a stable format for archiving of RAW images (e.g. to write-once media) that does not by itself imply that your “working” files need to also be converted to DNG. And if your archives are on easily-accessed media such as hard drives (even if spanning multiple terabytes) then it may be just as easy to convert them to DNG as-needed in the future (e.g. if support for old files was somehow going to be dropped). I think it’s highly unlikely that Adobe will drop support for older file formats, and if they do you’d probably hear about it with plenty of opportunity to convert affected files. There’ll always be third-party tools (including open-source tools) to allow access to those files.
- Creating DNG files takes time. It’s usually not efficient to convert RAWs to DNG as they’re being ingested/imported, although conversion can happen later.
- Metadata (keywords, etc) are stored within the DNG file instead of in a sidecar XMP file. This means any change to these will trigger many backup systems to copy the whole DNG file instead of a tiny XMP file. The disadvantage here is speed. Of course the same issue applies to JPEG/TIFF/PSD files anyway. Note that a single change to your keyword hierarchy in Lightroom can change the inherited keywords for thousands of images.
- Unless you embed the original RAW file within the DNG (which does away with any size advantage) or maintain separate archives of the original files, you will not have the option of processing the files through non-DNG-capable software (such as Canon’s and Nikon’s own software) in the future. This might or might-not be an issue for you.
Some people would list the embedding of metadata within the file as a “pro” for DNG instead of a con, and that seems to be because they’re afraid of losing track of XMP files. Any reasonable management software will move/rename/etc these files alongside the matching RAW file. Both Bridge and Lightroom do this for example. Or maybe it’s because they’re looking at folders with tools such as Finder or Windows Explorer (neither of which are good tools for managing folders of images anyway) and getting “offended” by all the extra XMP files. It’s always been a non-issue for me.
You can draw your own conclusions, but I’ll tell you about my own workflow:
- I do not convert RAW files to DNG as they’re being imported. The speed trade-off is not worth it. Currently I do not do the conversion until the files are on my office systems. If I’m on an expedition with 300+ GB of RAW files, I like to be able to save metadata from within Lightroom to have the XMP files as additional backup. With sidecar files this keeps the backup synchronisation in the field reasonable without having to wait for all 300 GB to be backed up again.
- I do convert files to DNG later, but currently this is purely to give me the peace of mind of the internal checksums. I’m looking forward to having decent bulk checksum-verification tools.
Years ago I converted all my images to DNG as it saved me hundreds of MB of storage. These days this issue doesn’t worry me so much: although my file sizes have got bigger with newer cameras, disks have also increased in size and decreased in cost.
The other reason I did this conversion was because I was using Expression Media to manage my images (and ACR to process them) and the preview/thumbnail issue meant I could see in EM what the image “really” looked like. That issue went away when I started using Lightroom.
Hopefully this post helps you make your own decision about the use of DNG!
All the photos in this post are 10 years old, taken in RAW with an EOS D30.