A student asked a question (you know who you are, Alan), that I thought I would answer here.

But there is not much to answer, since Alan has masterly answered most of his question himself. Here is his question in its entirety (except I removed the name of a book):

I’m learning Lightroom – what a fascinating system. It’s going to take some time to get used to having it manage my photos, instead of doing it myself. But I can see the benefits. Thank you again for recommending it.

Now — what about the whole RAW vs. DNG thing? The book I purchased, “[Book Title]”, strongly and repeatedly recommends converting the CR2 files to DNG, and then discarding the CR2.

I understand that DNG is just another type of RAW file, but it makes me nervous:

1. CR2 is, well, really RAW. It’s the original file.

2. A “benefit” of DNG is that it doesn’t have the separate XMP sidecar files, but instead writes the sidecar-type metadata into the DNG file. That means that every time I edit a picture, I’m editing the DNG file. What if that leads to corruption? The constant modification of the DNG seems contrary to the philosophy of never altering the digital negative.

3. Since the DNG keeps changing, that means that my Time Machine backups will have to keep backing large DNG files. By contrast, the CR2 file is backed up once, and never touched again. The only thing that Time Machine has to backup is the very small XMP.

I have hundreds of CR2 raw files (I only keep the good ones, maybe 10-20 per shoot), and am really unsure what to do here. (I searched your blog, didn’t find this issue addressed.) Do you stick with the CR2 files, or convert to DNG?

I removed the name of the book because this way I feel free criticising its advice.

Alan is right. All his arguments are spot on.

And let’s expand the first one: a RAW file is the original negative, and a DNG file is an interpretation of that file. It is not a straighforward process. You have to interpret the CR2 file and make it into a DNG using your interpretation of what the bits mean.

Because of that and the other reasons Alan has worked out, my advice is to keep your original RAW files and not to convert them to DNG.

8 thoughts on “RAW or DNG?

  1. But since Lightroom’s edits are non-destructive, you’re not really editing the DNG file any more than you are the CR2 file, right? Not sure what he means about the DNG constantly changing, is he going back and tweaking it a lot? I have tens of thousands of DNG files and I have never suspected that Time Machine wastes any space with them – they’re pretty similar in size to the CR2 files.

    Maybe I’m missing something, but it’s my understanding that DNG is not an interpretation of the file. You can convert your RAW files to DNG upon import, then go back into Lightroom and start over if you have to, just like with CR2. The supposed advantage is that you have one type of file that is designed to be more future-proof than the various RAW formats.

  2. Well – that is debatable: when cameras all shoot DNG, fine. But even then, CR2 will always be around, billions of photos having been taken in this format. And will Canon ever give up the ability to tune and tweak their own format? It’s what they “do”. Commoditizing that format will not sound like a great idea to the major camera makers. So it may or may not happen. Adobe would like it to, sure. But whether it will, I am not sure.

    You may be right about Lightroom not editing the DNG files, but then the supposed benefit of embedded settings is lost.

    In any case, to me the most important issue is the interpretation. There is certainly some interpretation going on. There is no one way to translate the bits. While some translation is one to one (EXIF settings, for example), in areas such as colour spaces, dynamic range, white balance: every time you create an image you are certainly making choices (and they are Adobe’s choices, not all of which are fully documented). There is only one real original.


  3. Yeah, you make a good point about proprietary RAW files. It’s inconceivable they’d become obsolete (especially Canon and Nikon files). And you’re right about there only being one true original. If there’s any interpretation going on upon conversion to DNG, however, I have yet to see it.

    Personally, I’d love it if Canon, Nikon, Samsung, et al would just adopt DNG as the standard (as Leica has done). I test a lot of cameras as they come out and it can be frustrating waiting around for another RAW update before I can play with the RAW files…I am not a fan of shooting in JPEG. I am not fond of camera makers’ supplied software, though it does provide a temporary workaround. Thanks for your reply! This is an interesting topic of discussion to me.

    • Indeed, it would be great if there were one format. I support the idea wholeheartedly.

      And yes, there is interpretation. Since the formats of the files are not identical, you have to decide how to convert format a to format b.

      Add to that the fact that Adobe has to reverse-engineer the RAW formats, and make decisions based on that. And no doubt it fails to convert some of the info (almost by definition, it cannot be a superset of ALL raw formats).

      (Try this. Open a RAW file in several Canon programs. You will see a different interpretation in each case! EOS Utility interprets a Canon RAW file slightly differently from Digital Photo Professional: the colours, etc are slightly different. And that is two interpretations from one maker…)

  4. Thanks, Michael! 🙂

    I’ve decided to stick with RAW for the time being… the sidecars have never bothered me, and getting rid of them seems to be the only real benefit of DNG. Thus, I’ll stay with the CR2 files on my EOS cameras, and the RW2 files on my wonderful new Lumix LX5.


  5. Great discussion, Michael! Thanks for hosting this conversation on your blog.

    Though Alan’s statements might be valid, in my workflow, they are irrelevant. For one thing, I don’t care how long it takes to sync my backups; I do it overnight using Chronosync.

    I also want to point out that the statement “a RAW file is the original negative, and a DNG file is an interpretation of that file. It is not a straighforward process. You have to interpret the CR2 file and make it into a DNG using your interpretation of what the bits mean” … this is wholly incorrect.

    When converting a proprietary raw file to DNG, all of the original raw data is preserved in its original state within the DNG file. There is no “interpretation” or conversion of any kind being done. The raw data is simply placed inside a metadata “wrapper”. A raw file and a DNG render exactly the same in Adobe software.

    BTW, it’s my book that’s called to task here. And it’s true that I am a big proponent of DNG for the reasons I quote in my book (and not because Adobe sponsors me to say this).

    And it’s also true that I don’t keep my original raw files after conversion to DNG. I have never regretted it. I have 60,000 raw captures in my library and they are all DNG. Needless to say, I haven’t experienced any quality issues whatsoever using DNG.

    DNG is my raw “negative” format of choice, and I’m very pleased with it. But like most everything else in the digital photography workflow, it’s a personal choice. Do whatever it is that helps you sleep at night 😉

    There is no advantage to sticking with CR2, NEF, etc. whereas DNG offers several significant benefits, at least in my workflow.

    If you want the real story about DNG go to http://www.adobe.com/dng and check out the PDF papers.

    • Ah, this opens up a whole new discussion area. Thanks for coming in on this.

      I am off to speak at a show for three days, but will be back on this after the weekend. In fact DNG is a wrapper around a TIFF file. Original data may be embedded; Some tags may be lost. It’s complicated.

      Again, more after the weekend. Thanks for the link: readers can read the spec PDF meanwhile to learn more.

      Now I wish I could get back to sleep. Three days’ public speaking to do.

Leave a Reply

Your email address will not be published. Required fields are marked *