What is actually inside a DNG
DNG is Adobe's open raw container, and Apple's ProRAW writes it too. Inside there are three separate things: the sensor data, a pile of metadata, and one or more JPEG previews that let a file browser show you a thumbnail without doing any raw processing.
The sensor data is not a picture. It is a grid of single colour brightness readings behind a colour filter array, and turning it into an image means demosaicing it, applying a white balance, applying a colour profile and applying a tone curve. That is what Lightroom does, and it is not something a web page should pretend to do.
The preview is a picture. It is the camera's own interpretation of the shot, encoded as a normal JPEG and stored in one of the file's image directories. This tool walks the TIFF directory structure, finds the largest JPEG in there, and uses that.
What you gain and what you give up
| Embedded preview | Developed in Lightroom | |
|---|---|---|
| Time per file | Under a second | Minutes, plus a subscription |
| Resolution | Usually full, sometimes reduced | Always full |
| Your edits included | No | Yes |
| Highlight recovery | No | Yes, that is the point of raw |
| Good enough for a proof sheet | Yes | Yes |
| Good enough for a print | No | Yes |
The contact sheet use case
Say you shot 60 frames and want the client to pick ten. Exporting 60 JPEGs and zipping them means the client has to download, unzip and open a folder. A single PDF opens in their mail app, scrolls, and prints if they want it on paper.
Set page size to US Letter or A4 so every sheet is the same, drop the files in, and check the order. The page numbers in the list are the page numbers in the PDF. Then send one file instead of sixty.
When this will not work
Some raw converters strip previews when they write DNGs, to keep archive files small. Some third party camera apps only write a small thumbnail. And a handful of proprietary raw formats store their previews in ways that are not TIFF standard at all.
In all of those cases you get a named error rather than a broken page, and the fix is the same: open the file in whatever you normally use, export a JPEG, and bring that to JPG to PDF.