Reading through DNG specs, I learned (or at least that was my understanding of) that private maker notes data is always preserved, even if it is not understood by the DNG format.
However, these days I was playing with ExifTool and, while looking through Nikon D70/D80 NEF files, I could see the FocusDistance tag. This is a private Nikon tag and I couldn't find it in converted DNG. Knowing that DNG should keep everyting, I went to ExifTool's web-site to find where in the DNG this tag might have been stored.
I found the following statement on ExifTool's web-site:
The maker notes ('MakN') are processed by ExifTool, but some information may have been lost by the Adobe DNG Converter. This is because the Adobe DNG Converter (as of version 4.3) doesn't properly handle information referenced from inside the maker notes that lies outside the original maker notes block, and this information is lost when only the maker note block is copied to the DNG image.
So, where's the truth? And where is my FocusDistance data in the DNG?
There's an inherent problem with the MakerNotes field in EXIF format;
EXIF uses what is in effect relative block addressing, so unless you
know what is actually in the MakerNote, you may not be able to copy it
safely (in order to copy it, you may have to adjust block addresses,
and if you can't decipher the MakerNote, you can't adjust the
addressing). So the block as a whole "blob of data" will be copied if
the Adobe Converter understands it, but it may not be possible for
software that later modifies the DNG file to safely copy it forward,
unless either the software in question understands the MakerNote, or
it has no addresses. That's why the DNG format has the
Private EXIF tags are a different issue; the Adobe folks would need to
comment on that.