What Is a Sidecar XMP File?

Metadata

A companion file

A sidecar XMP is a small text file carrying the same name as your image with an .xmp extension. It accompanies the original without modifying it: _DSC4471.NEF on one side, _DSC4471.xmp on the other.

It exists for a specific reason. Proprietary RAW files — NEF, CR3, ARW — are closed formats their vendors advise against rewriting. Rather than risk altering an irreplaceable original, applications drop the metadata alongside it.

What it holds

XML, in the XMP format defined by Adobe and widely adopted. Typically:

• Descriptive metadata: caption, title, keywords, location, creator, rights.
• Curation decisions: rating, colour label, pick or reject.
• Develop settings from the editor that wrote it — crop, exposure, curves.
• Application-specific blocks that other tools ignore without understanding.

The two rules that stop you losing work

Rule one: the sidecar travels with the image. A sidecar separated from its file is an unreadable orphan. Copying, moving, or archiving one without the other silently destroys hours of description. Check that your copy and backup tools carry the .xmp files.

Rule two: never overwrite, merge. A sidecar may hold blocks written by an application other than yours. A tool that rewrites the whole file instead of merging its own fields into it erases those blocks without warning. This is the most common cause of metadata loss when switching tools.

Sidecar or write into the original?

For a proprietary RAW, the sidecar is the prudent choice. For a DNG, TIFF, or JPEG, writing directly into the file is possible and removes the orphan problem — at the cost of modifying the original.

Many serious collections do both: sidecars for RAW, internal writing for derivatives, and a safety copy of the inventory in the catalog.

In practice

1. Confirm your backups include .xmp files.
2. Never rename an image without renaming its sidecar in the same action.
3. Before adopting a tool, test that it merges instead of overwriting.
4. Keep a copy of your metadata in the inventory, independently of the sidecars.

Obscura Flow merges XMP sidecars instead of rewriting them — blocks it doesn’t manage are preserved verbatim, so passing through it doesn’t cost you annotations made elsewhere.