Nick & Sven,
The issue here of course is that some applications (like Windows Explorer or Mac Preview) use the Exif/TIFF "Image Description" tag for the caption rather than using the IPTC or XMP "Caption" field. I believe the same may be true for Exif/TIFF "Artist" being used instead of IPTC/XMP "Byline" (or Photographer) and Exif/TIFF Copyright being used instead of IPTC/XMP Copyright.
PM hasn't updated these Exif/TIFF tags "in-place" in a JPEG because of the dangers of relocating Exif data when its size increases. The danger is that any camera manufacturer "maker note" could potentially be "broken" if tag data are relocated within the file, thereby changing the actual offsets of data without updating the proprietary maker note information that stores the offsets to maker note fields.
When applications like Photoshop save out a new image, the maker note is deleted and the Exif tags are written with the updated IPTC/XMP values. PM does the same thing on a SaveAs to JPEG except in the case where a JPEG is saved from a JPEG in which case the entire Exif block is simply copied to the new JPEG.
So to fix this problem I have decided to just delete any of the 3 tags Exif tags (Image Description, Artist, and Copyright) when they are present in a JPEG when PM updates the IPTC/XMP. This causes these programs to instead find the correct Caption etc within the IPTC or XMP.
The fix will be in the next release.
--dennis
