Previews and metadata
DNGs will [?]
contain a preview JPEG (tiny, medium or full size) and all the metadata of the original raw file, plus any metadata you want to add such as keywords, titles, captions or develop settings. For anyone using Adobe Lightroom, Bridge or Camera Raw this means that you’ll have one DNG file instead of a CR2/NEF image file and an XMP metadata file next to it.
Also, when you’ve changed the appearance of a photo you can render a new preview JPEG and embed it in the DNG. (In Lightroom: Right click and choose Metadata -> Update DNG Preview & Metadata.) I don’t know how NEFs work, but with CR2 files there is definitely no way of updating the embedded JPEG.
DNG files are often smaller then the camera’s raw file. This is thanks to better compression, but also thanks to the possibility to use a smaller size JPEG than in the original.
I chose the medium JPEG size (1024 pixels I think) and the DNG files are on average 19 percent smaller than the CR2s from my Canon EOS 50D. My thinking here is that I don’t really need a full size preview in the raw file once I have it on the computer. In the camera it’s a different matter – there I need the full size preview to be able to check focus. And even if I do choose to embed a full size JPEG the DNG ends up ~10% smaller.