LightRoom 4’s New DNG Spec Allows Lossy Compression

Adobe Photoshop LightRoom 4 has dropped, and I must say it came faster than I expected.  This is good news of course, along with the 50% price cut.

Anyhow, there’s plenty of coverage of the software ALL OVER the ‘net, so you don’t need me to tell you about it yet again.  What I am going to do is target very specific things that I find useful.

Today’s tidbit is about the new DNG format.  Adobe LightRoom 4 introduces the DNG 6.6 spec.  DNG 6.7 is coming very soon, and adds support for the Canon 5d mark III and a handful of other brand-new cameras.  If you need 5d mark III support today, you can download the “release candidate” or “RC” version of DNG 6.7 from Adobe’s website.  (A “release candidate” has passed beta, and is the version the software engineers believe will become final.)

What’s so fancy about the 6.6+ spec?  Well it supports lossy compression, for one.  This is similar to the way a JPEG is compressed and gets those nice small file sizes.  If you’re like me, your hard drives are packed full of huge DNG files, even though the majority of those files never actually go to the client.  You feel obligated to store them just in case, but it seems silly for them to take up so much space.  Converting them to JPG is certainly an option, but that locks you out of a lot of the cool features of a RAW file.  What to do?

Enter lossy DNG.

In my workflow, everything with three stars or more is delivered to the client.  One and two star files are kept, but not delivered, and I never waste time color balancing or exposure correcting them.  This is the benefit of the lossy DNG as a solid middle ground.  I save a lot of disk space, but I don’t discard the ability to post-process them.

How much disk space would I save?  This is hardly a scientific survey, but as a test I converted one lossless DNG (from the previous 5.5+ spec) to a lossy 6.6+ spec.  It went from 23.4MB to 9.6MB – a savings of over half.  Repeat that across enough files, and we’re talking about some serious storage savings.

So what’s the easiest way to implement this?  The quick-‘n-dirty answer is “attributes: less than or equal to two stars (three stars is my lowest rating for client delivery;) metadata: date: appropriate date range,  file type: Digital Negative / Lossless.”

Adobe Lightroom 4 “Attribute” and “Metadata” filters for selecting only reject photos to convert to lossy DNG.

Then in the grid select all and convert the selected photos to DNG like this:

Lightroom 4 “Convert Photo to DNG” options window.

If you didn’t quite get all that, or if you’re wondering why I made some choices, let’s dig a bit deeper.

  • Filtering
    • I deliver files with three stars or higher.  Setting less than or equal to two stars shows only the images that didn’t make the cut.  I don’t want to convert 3+ star files.
    • Filtering by date is up to you.  There are a few projects from a few months back that I’m still working on, so I won’t make any files lossy just yet.
    • Filter by File Type.  You’ll have to pull the mini-menu at the top of the column down to access this one, as it’s not shown by default.  Then select “Digital Negative / Lossless.”  This will narrow the set of displayed images even further to only DNG files without compression – convenient for filtering TIFFs, and for later on when you want to update more recent projects without re-converting files already compressed.
    • If you’re converting RAW files that are not yet DNG files, you’ll also need to select the RAW option in the File Type category.

Now that we’ve got all the appropriate files showing in the library grid down below, select all of them
and go to the menu Library -> Convert Photos to DNG…   You’ll get the window shown above.

  • Converting to DNG
    • Only convert RAW files:  We’ve already constructed a filter so that only RAW files are selected – I just leave this on by default.  If we hadn’t filtered, this would prevent it from trying to handle non-RAW formats.
    • Delete originals after successful conversion:  This piece is key.   If this is not enabled, it will keep the original large file, and the new compressed file.  By enabling this, we tell Lightroom to make the new compressed file, substitute it in to the place of the original, and discard the original.
    • Compatibility:  This must be “Camera Raw 6.6 and later” in order to access the lossy options.  Previous versions do not support it.
    • JPEG Preview:  This is up to you.  When this is on, my computer’s file browser shows a thumbnail of the file rather than an icon of the file.  Further, if I know that I’ll be browsing the RAW files with PhotoMechanic, this must be on or PM won’t show anything.  (It’s speed comes from the fact that it reads the preview data and doesn’t process the RAW.)  I turn this off in this case because it saves 2 – 3MB per file on my 5d2, and I know that I’m unlikely to ever go back to these files.  This option has no impact on Lightroom’s ability to show previews.
    • Use Lossy Compression:  On, because that’s why we’re here today.
    • Embed Original RAW File:  This is probably only important to museum archivists.  The rest of us should turn it off.  It will make file sizes double.

Done!  Now it’s time to just let it roll.  I’m converting over 47,000 files, and so far Lightroom has been cranking away for about 10 hours and is about 60% complete.  Start this thing working, then go to sleep.

BTW, the new DNG spec also supports down-sampling so one could reduce the files to say, 10 megapixel instead of 21.  That functionality is not yet exposed in the Lightroom converter, though, so I haven’t played with it yet.

What do you think of the new lossy option in DNG?  Will you be using it?  Questions and comments below!

2 thoughts on “LightRoom 4’s New DNG Spec Allows Lossy Compression”

  1. I always understood the advantage of DNGs was having digital negatives, that were unaltered. If you're losing data, and quality, that seems risky for future use.
    With the cheap price of data storage, I think I'll keep mine in straight, lossless DNGs.

  2. Thanks for stopping by and leaving a comment, Dave. I hope you'll keep coming back! This is in no way meant to be a solution for every photographer, but for some(myself included) it's a useful tool. To anyone else reading it bears repeating – I would NOT advocate this practice for files you deliver to your clients. Cheers!

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