Today’s Question: If I haven’t had the auto write XMP option turned on [in Adobe Lightroom Classic], will turning it on only affect photos after I turn it on?
Tim’s Quick Answer: No, when you enable the option to automatically write metadata to your photos, Lightroom Classic will update all existing photos as well. It may take a little bit of time for the task to complete, but it will be done automatically in the background.
More Detail: Lightroom Classic includes an option to automatically write supported metadata values to the actual image files, so that most of your metadata will be stored with the actual images that data relates to. The information will be stored in an XMP “sidecar” file for proprietary raw captures, and within the actual file for other image formats.
You can find the “Automatically write changes into XMP” checkbox on the Metadata tab of the Catalog Settings dialog. To bring up the Catalog Settings dialog, choose “Catalog Settings” from the Edit menu on Windows, or from the Lightroom Classic menu in Lightroom.
The checkbox is turned off by default, so that metadata updates are only stored within the Lightroom catalog. That means other applications won’t be able to see the updated metadata for your photos, and that you don’t have the benefit of an automatic backup of some of the most important data about your photos.
When you enable this option, Lightroom will go back and update the files for all existing images, so that once you’ve allowed enough time for the processing to complete (which generally will only take a few minutes), all existing images will have had their metadata updated on the hard drive, in addition to still being included in the Lightroom catalog.
It is important to note that Lightroom-specific features will not be saved to metadata, because they are not part of an established metadata standard. The means the metadata updates will not include Pick and Reject flags, membership in collections, virtual copies, the history for your photos, and other features that are specific to Lightroom and therefore only stored within the Lightroom catalog.