In discussing non-destructive JPEG editing in Lightroom and Camera Raw, I mentioned that it’s possible to convert JPEG files into DNG–a format previously limited to raw data from camera sensors. Why do the new tools allow this, and why might it be useful? Here’s some perspective from Tom Hogarty:
It’s been almost a year since Lightroom introduced the ability to convert TIFF and JPEG files to the Digital Negative (DNG) format. This does not mean that Adobe is magically converting output-referred TIFF/JPEG files into mosaic data that has all of the flexibility of native raw files. These converted JPEG/TIFF files are not raw files at all.
So, why allow the conversion?
As Lightroom and now Adobe Camera Raw provide non-destructive editing of JPEG and TIFF files, the DNG format offers benefits as a non-destructive editing format in addition to its position as a raw standard. DNG is designed to efficiently store the XMP metadata block and image preview associated with a non-destructive edit. As non-destructive editing capabilities grow, the DNG format has the architecture required to grow with those capabilities regardless of the source format. For example, a JPEG image converted to DNG and non-destructively edited three different ways will be able to store three sets of editing instructions and three distinct previews for each edit.
Does this lessen DNG’s position as a raw format standard? Absolutely not. The core of public DNG specification is a standard method of storing and describing raw data. Most recently, Leica and Pentax have joined the ranks of camera manufacturers supporting DNG files natively and there are a substantial percentage of professional photographers converting their proprietary raw files to DNG for workflow or archival purposes.
So, editing a JPEG in Lightroom or ACR, then making it into a DNG, allows you to create an envelope that packages up the original bits, the editing sauce, and a rendered preview that any application can see (i.e. DNG = before + after + settings). And, unlike a regular JPEG that contains editing data, a DNG isn’t going to be mistaken for any old file. It stands out as something with special editing properties.
Having said all this, converting JPEG to DNG is useful, but it’s not a panacea: it makes files larger (at least for now), and it’s not something I think everyone should run out and do. (I haven’t found a need to do it myself.) It’s an option, however, and one that could grow more useful in the future.