Free-text notes in EXIF go in UserComment (0x9286), a tag in the Exif sub-IFD meant for remarks by the photographer or by software rather than a published caption. The odd part is the encoding. Its type is UNDEFINED, and the first 8 bytes are a character code that tells the reader how to decode the rest:
| Code | First 8 bytes | Text that follows |
|---|---|---|
| ASCII | 41 53 43 49 49 00 00 00 | ASCII |
| UNICODE | 55 4E 49 43 4F 44 45 00 | UTF-16 |
| JIS | 4A 49 53 00 00 00 00 00 | JIS X 0208 |
| Undefined | 00 00 00 00 00 00 00 00 | not specified |
ExifTool picks the code for you. Plain ASCII text gets the ASCII header; one accented letter switches it to UNICODE. Verbose output, two test files:
| | 2) UserComment = ASCIIShot from the north pier
| | - Tag 0x9286 (32 bytes, undef[32]):
| | 00ba: 41 53 43 49 49 00 00 00 53 68 6f 74 20 66 72 6f [ASCII...Shot fro]
| | 2) UserComment = UNICODEQuai nord caf.
| | - Tag 0x9286 (36 bytes, undef[36]):
| | 00ba: 55 4e 49 43 4f 44 45 00 00 51 00 75 00 61 00 69 [UNICODE..Q.u.a.i]In the UNICODE case the UTF-16 follows the byte order of the EXIF block, big-endian in this file (00 51 is "Q"). ExifTool's normal output hides the header and prints Quai nord café.
Where readers trip
A reader that ignores the header shows the comment with ASCII or UNICODE glued to the front, as in the verbose lines above. One that assumes little-endian UTF-16 turns a big-endian comment into CJK-looking garbage.
Windows keeps a separate comment tag, XPComment (0x9C9C), stored as UTF-16 in IFD0. Editing one doesn't change the other, so a comment typed in one program can be invisible in another.
Changing the comment
UserComment is in our whitelist: the EXIF editor shows and writes it on JPEG and TIFF. The usual character restrictions apply (no commas, colons or semicolons). With ExifTool, exiftool -EXIF:UserComment="Shot from the north pier" photo.jpg sets it and -EXIF:UserComment= deletes it.
For a caption you want other people or a CMS to see, use ImageDescription instead. The EXIF data guide covers the rest of the Exif IFD, and the JPEG page shows where that IFD sits in the file.