They’re wrong.
Lightroom Classic edits are stored separately from the original data, especially if the original is a raw file. The only way edits are permanently applied is when you export a copy.
By default, Lightroom Classic edits are stored in a catalog. But a single image can be referenced from multiple catalogs, like your laptop and home catalogs, so now you have to understand what happens in that case.
Edits aren’t cumulative across catalogs. So you won’t get double sharpening from applying sharpening in two catalogs. What can happen, depending on what you do, is that an image’s sharpening value in a catalog can be replaced (not added to) from another catalog or an XMP file. In other words, you can get edit conflicts between catalogs and XMP files.
When you view an image file in catalog A, you see the edits catalog A stores for that image.
When you view the same image in catalog B, you see the edits catalog B stores for that image.
The image file itself is unchanged, because the edits are in each catalog, not in the image.
Therefore, you don’t have double sharpening, because both catalogs are not permanently applying anything to the image itself and not interacting with other catalogs.
Now suppose you’re also using XMP files. Lightroom Classic replaces image edits in the current catalog with edits from XMP files only when one of these two things happens:
- You import an image that has an XMP file.
- You select the image in a catalog and choose the Read Metadata From File command.
Either of those replaces the edits in the catalog.
OK, so what happens if you have two catalogs referencing the same image AND there is also an xmp file for that image?
When you view an image file in catalog A, you see the edits catalog A stores for that image.
When you view the same image in catalog B, you see the edits catalog B stores for that image.
After import, the XMP files don’t affect any catalogs unless you manually read them into a catalog.
If you select an image in catalog A and use the Read Metadata From File command, then the edits in the XMP file replace the edits in catalog A but not in catalog B, only in the open catalog.
If you select an image in catalog A and use the Save Metadata To File command (or if Automatically Write Changes Into XMP is enabled), then the edits in Catalog A replace the edits in the XMP file, but not in catalog B unless you also open catalog B and use the Read Metadata From File command to replace its edits with the ones in the XMP files.
If you want the same edits to be in both catalog A and B, you could have one catalog write its edits to XMP and then have the other catalog read from that same XMP files. But it can be better to instead use the Import From Another Catalog command, because although XMP files can store Develop module edits, XMP files don’t store catalog-level edits like which collections, slide shows, and print jobs it belongs to. But none of this is automatic, moving edits across catalogs is always something you have to do manually.
As long as you pay attention to which direction you’re reading and writing the metadata, you can control which way the edits flow.
And it’s a good skill to master, because if you do, you’ll understand exactly how to move image edits not only between catalogs and XMP files, but also how to use XMP files to transfer Lightroom Classic edits to and from any other apps that read XMP files, such as Adobe Bridge, Adobe Camera Raw, Adobe After Effects, and non-Adobe software like LRTimelapse.
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