Today’s Question: ACR files have recently started appearing on my hard drive next to original JPEG files. I edited the JPEGs in Lightroom Classic and I have not yet exported any of the edited files. Is there a reason for the appearance of the ACR files? Could I safely delete the ACR files on my hard drive without losing any of the edits made in Lightroom Classic?
Tim’s Quick Answer: The ACR files (with a filename extension of ACR) that you’re seeing are supplemental sidecar files that contain some of the more extensive edit metadata, such as masks and distraction removal. While they can be safely deleted, I don’t recommend doing so as they provide a backup to the information from the Lightroom Classic catalog.
More Detail: As of version 15, Lightroom Classic will store some metadata in ACR sidecar files in addition to XMP sidecar files, when metadata is saved to the source image files rather than only the catalog. This is in part to reduce the file size for XMP sidecar files.
When Lightroom Classic determines that there is a relatively heavy metadata load for an image, it will save some of the more significant edit-related metadata to an ACR sidecar file rather than the XMP sidecar file. This includes more substantial edits such as masks for targeted adjustments, AI features including distraction removal, Super Resolution, and Denoise.
XMP sidecar files are only created for proprietary raw captures, while standard metadata for other supported file types is saved directly into the source image. The metadata stored in ACR files is saved in an ACR file for all supported file types, which is why you might see ACR files for JPEG images, for example, in addition to proprietary raw captures.
Note that by default all updates are only saved to the Lightroom Classic catalog. The metadata referenced above will only be saved to the source files on the hard drive if you have turned on the “Automatically Write Changes into Sidecar Files” checkbox on the Metadata tab of the Catalog Settings dialog, or if you manually save metadata to files using the Metadata > Save Metadata to File command.
It is also worth noting that Camera Raw in Photoshop also uses the ACR sidecar files, so even photographers who don’t use Lightroom Classic may see these ACR files start to appear.

