I spent the last days cleaning up my photos and Lightroom catalogs. While importing/exporting and updating I noticed that the Pentax DNGs are uncompressed (around 17mb per file) and if I import them using the Lightroom “convert to DNG” command, they are compressed to about 9mb each.
However, using the existing “Convert Photos to DNG” command will not compress the existing DNGs in the library!
This is found under the “Library menu in Lightroom 3” and works great with Nikon or Canon Raw files. However, DNGs are ignored by this command.
You won’t have to re-import all photos to get the benefit of compression though.
There’s a solution which is not very obvious but worked very well for me:
Select the photos in your catalog and choose “Metadata -> Update DNG Previews & Metadata”
This will take the uncompressed DNGs and compress them, giving you between 10%-40% of space back!