In the past when discussing Adobe’s DNG format in Basic Lightroom classes and books, I would say converting to DNG during import poses little risk. No data is lost and the DNG format has all the Lightroom editing capabilities of the original raw format. I recommended this because an advantage of DNG is that Lightroom can write metadata directly to the file, eliminating the need for XMP sidecar files with proprietary raw formats.
The main downside to converting proprietary raw files to DNG during import is that the original raw files are normally discarded. I never considered this as a major issue because no important data is lost during the conversion. But yesterday a reader from England told me about a situation with DNG files that changed what I’ll be teaching in the future.
Conor explained that he was planning to enter some photos in a wildlife photography competition. The rules for the competition stated that if your work is selected, you will need to provide the original raw file of the image to show it hasn’t been edited in certain ways, such as compositing. The rules also stated that DNG files would not be accepted as original files. The reasoning behind this is that proprietary raw files (such as CR2, NEF, ORF, etc.) that come directly out of a camera cannot be permanently edited. You can always revert the file back to its original status. But this isn’t true with some DNG files.
Though some cameras are able to capture raw files in the DNG format, you can also create DNGs outside the camera. For example I can convert a heavily edited TIFF to the DNG format. Due to this flexibility a DNG file cannot be trusted to be a representation of an original file. This is why Conor ran into the problem with the wildlife competition. I imagine DNG is also an issue when entering photojournalism competitions and when presenting photos as evidence in legal proceedings.
These situations may apply to only a handful of photographers, but they are reason enough for me to change my point-of-view on DNG. For now on I’ll tell students to forego the DNG file format and keep their original raw files intact. Yes, you can embed the original proprietary raw file inside a DNG when converting during import, but that will make the file almost twice as big as the original. I don’t see any advantage to DNG that would justify doubling the size of all your raw files.
One more thing. Be sure to go to the Metadata tab in Lightroom’s Catalog settings and select the option to Automatically Write Changes to XMP, as shown in the figure above. This will ensure Lightroom’s metadata is saved with the original raw file rather than being buried in Lightroom’s catalog. This way if something unexpected happens and corrupts your catalog and you don’t have a good backup, all your edits are with the original files.