On Wed, 2018-04-04 at 13:36 +0200, Tobias Ellinghaus wrote:
> Am Mittwoch, 4. April 2018, 13:15:39 CEST schrieb Timur Irikovich
> Davletshin:
> > Hello everybody!
> > 
> > Just decided to play with DNG converter and discovered problem
> > related
> > to lens data interpretation by DT (2.4.2).
> > 
> > 1. NEF file loaded by DT applies correct lens parameters.
> >     Lens info shown by DT: Nikon AF-S DX Nikkor 18-55mm f/3.5-5.6G
> > VR II
> > 
> > 2. DNG file cannot apply it because wrong EXIF information is used
> > (short lens name vs. full).
> >     Lens info shown by DT: 18.0-55.0 mm f/3.5-5.6
> > 
> > 3. Focus information also is lost in DNG.
> 
> That's why we tell everyone to never convert to DNG.
> 
> Tobias
> 
> [...]

So, after little digging I found following (correct me if I'm wrong):

1. DT relies only on libexiv2 in extraction of EXIF metadata.
2. exiv2 fails to extract vendor specific information from DNG file
(converted via Adobe DNG Converter) when compared with exiftool. But
original NEF data is there.
3. As last resort for lens detection DT relies on Exif.Photo.LensModel,
which is just ASCII string encoded by converter.
4. Converter writes ambiguous data in Exif.Photo.LensModel without. No
vendor, no exact model, just '18.0-55.0 mm f/3.5-5.6'.
5. Feeding vague data to lensfun gives no good results.
6. Native DNG by Pentax use non-standard vendor specific Exif.Pentax*
namespace.
7. Focus distance information is also vendor specific.

If not mistaken problem should be fixed on very root... which is
libexiv2. But it's just MHO.
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