The "lenstype" is stored in the maker note which doesn't survive when saving a TIFF to a JPEG in PM. PM will, however, preserve the entire Exif block, which includes the maker note, when doing a Save As from JPEG to JPEG. But most applications don't preserve this maker note info. For example, Photoshop rebuilds the Exif block on a JPEG when saving, much like PM does when saving a JPEG from a TIFF or PSD, and the maker note is NOT included because it is a proprietary chunk of data. Although people like Phil Harvey of Exiftool have been able to decipher certain parts of certain maker notes for certain cameras, nobody but the manufacturer (I don't think) can create a complete maker note from scratch that is valid. The only safe way is to copy the entire JPEG Exif chunk in whole, but the Exif section in a TIFF file can technically be spread out all over the place and isn't nicely contained.
Photoshop does write some Exif fields into the XMP including the lens. PM will pick this up (if missing in maker note) but it doesn't write it out into XMP. I suppose we could add this on write. I will investigate further.
--dennis
