Hi,

Try with:
exiftool -exif:FocalLength=50 MyImage.tif

Above works, but there's a problem I've noticed when using PSP X2 (v12.5). That is, PSP doesn't save Makernotes as it should (that is: Makernotes section is somewhat corrupt).
Because of that, exiftool will (by default) refuse to write into such image file. To make writting possible, you should use:
exiftool -m -exif:FocalLength=50 MyImage.tif
-which ignores "m"inor errors and thus writes metadata

Of course, you can delete complete existing Makernote section (it became useless anyway) and write FocalLength as shown in first example.

Greetings,
Bogdan