A sidecar file is a small companion file — conventionally the image’s base name with an .xmp extension — that stores metadata for an image whose own format cannot safely hold it. DSC_4821.NEF travels with DSC_4821.xmp, and the second file is where the keywords, caption, copyright and rating actually live.
In plain English
A JPEG has a standard, safely writable place to put metadata, so a tool can embed keywords and copyright directly inside the file. A RAW file does not. RAW formats are proprietary to each camera manufacturer, frequently undocumented, and rewriting one risks damaging the original negative — the single irreplaceable thing in a photographer’s archive. So the software leaves the RAW strictly alone and writes a separate XMP file next to it.
The consequence is that a RAW image and its sidecar are a pair, not a file and an accessory. Move the .NEF without the .xmp and you have moved pixels while leaving every keyword, caption and rights statement behind. It is an unglamorous detail that quietly decides whether a decade of tagging survives a folder reorganisation.
Why it matters in a DAM
Sidecars are the mechanism that makes metadata portable for RAW files. Metadata written into files — embedded in a JPEG, or beside a RAW in a sidecar — travels with those files into any future tool. Metadata kept only in an application’s own database stays behind when you leave, which is what turns a routine tool change into a rebuild.
This is why the sidecar question comes up long before migration day. In our testing, Daminion and ACDSee write on save; digiKam writes XMP sidecars properly once sidecar writing is enabled; Lightroom does it only if you switch on Catalog Settings → “Automatically write changes into XMP”, which is off by default. Eagle and Apple Photos largely do not write IPTC/XMP at all. The tools in that last group are not necessarily worse tools — but their metadata is not yours in the sense that matters.
Sidecars are also what let two applications share one archive without fighting. Daminion coexists with Lightroom precisely because both read and write standard XMP: the developer edits, the catalog organises, and the sidecar is the contract between them.
The one setting to check today: open your cataloguing tool and confirm it is writing metadata to XMP, not only to its database. In Lightroom that checkbox is off unless somebody turned it on. Everything you have tagged since installing it exists in exactly one place, and that place is a database file you do not control.
What a sidecar does not carry
One honest limit, and it catches people mid-migration. Keywords, captions, copyright and ratings are standard XMP: every serious application reads them. Develop settings are not. Lightroom stores its edits in the sidecar as instructions — exposure +0.3, this crop, that curve — and those instructions are meaningful only to Adobe software. Another tool reading the same sidecar will find the keywords perfectly and ignore the edit entirely.
So a sidecar makes your organisation portable, not your edits. Before switching tools, export finished JPEG or TIFF masters of the work you actually care about. The sidecar will carry the fact that an image is a backlit portrait of the CEO cleared for print until March; it will not carry the grade you spent an hour on.
Related terms
See it in action
Our DAM software for photographers ranking covers which catalog tools write sidecars and which trap metadata in a database, and the metadata fidelity ranking measures how many IPTC fields actually survive an export and re-import. The photo library organization guide shows the sidecar-first workflow end to end.
FAQ
What is a sidecar file?
A sidecar file is a small separate file, usually with an .xmp extension, that sits next to an image and stores its metadata: keywords, captions, copyright, ratings and edit instructions. It exists because most RAW formats are proprietary and read-only in practice, so writing metadata into the original is either impossible or risky. The image and its sidecar travel as a pair.
Why do RAW files need a sidecar when JPEGs do not?
A JPEG has a standard, safely writable metadata block, so keywords and copyright can be embedded directly in the file. RAW formats are proprietary to each camera maker and frequently undocumented; software that rewrites them risks corrupting the original. The sidecar sidesteps the problem by leaving the RAW untouched and keeping the metadata in a standard XMP file beside it.
What happens if I lose the sidecar file?
The image survives; its metadata does not. Keywords, captions, copyright and ratings live in the sidecar, so a RAW separated from its .xmp is just pixels. This is why sidecars must be copied, backed up and moved together with the images they describe — a folder sync that skips .xmp files silently discards years of tagging.
Do Lightroom edits transfer through a sidecar?
Only to Adobe applications. Develop settings are stored as instructions, not as an edited image, and other tools cannot reliably interpret them. Keywords, captions and copyright are standard XMP and travel everywhere; the edits are effectively Adobe-specific. Export finished JPEG or TIFF masters of work you care about before switching tools.
Which tools write sidecars properly?
In our testing, Daminion and ACDSee write on save, and digiKam writes XMP sidecars properly once sidecar writing is enabled. Lightroom needs Catalog Settings > 'Automatically write changes into XMP' switched on, because it is off by default. Eagle and Apple Photos largely do not write IPTC/XMP at all, which keeps your metadata inside their databases.