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.