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I shoot with my Pentax K10d in the DNG format. The camera produces the files in an uncompressed format. I would like to save some disk space by compressing all in a batch process too in Lightroom.

Can I compress all files in my lightroom library at once?

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According to Attila Acs:

You can compress new DNGs:

if I import them using the Lightroom “convert to DNG” command, 
they are compressed

You can also compress DNGs already in your Lightroom database:

Select the photos in your catalog and choose
“Metadata -> Update DNG Previews & Metadata”
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The Lightroom Queen explains how to make Lightroom apply lossy or lossless DNG compression to raw files (including DNGs) that are in your photo library:

  1. Select the images. (This means LR can only do this one folder at a time.)
  2. Go to Library menu > Convert Photos to DNG... [not a memorable command name for this purpose]
  3. In the dialog, pick:
    • Only convert RAW files [that includes DNGs]
    • Delete originals after successful conversion
    • Pick lossy or lossless compression as desired
    • Not "Embed Original RAW File".

Note: This Adobe forum thread explains what "Update DNG Preview and Metadata" does:

  1. It does not compress the image files.
  2. Maybe takes hours.
  3. Updates the preview that's embedded in the DNG so other programs (that don't process Lightroom's non-destructive edit list) can get at least a version of the edited image.
  4. Updates the metadata stored in the DNG file, although it's unclear from this thread if/when that includes keywords, ratings, color labels, and the edit list.
  • Note that the original question here isn't asking about lossy compression, at least not necessarily. Due to CPU limitations, the K10D could only save completely uncompressed DNG files — not even lossless compression. Newer Pentax cameras do not have this limitation. – Please Read Profile Jul 29 '18 at 8:30
  • @mattdm Thanks, I missed that point. So the original question is how to compress DNGs in the photo library, either lossy or lossless. I'll update that step. I found this question looking to further compress library photos applying lossless DNG compression during import, but the accepted answer didn't work. – Jerry101 Jul 29 '18 at 8:43

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