Comment on Exif Tool

Example: set caption from filename (useful before uploading to Picasa Web):
<code>exiftool -e -overwrite_original_in_place -UserComment="$file" "$file"</code>
($file is the name of the current file being processed by the shell script.)

This will set the caption in the iptc, while being lossless for the image.
(Aperture can do this operation via a script, but you have to export, ie: recompress the file, afaik.)

NB: some software use the UserComment exif tag for a description. Unfortunately, exif tool adds charset="Ascii" in front of the value when trying to write a new value into this field.