I recently had a philosophical difference on a forum website about removing and reimporting processed raw images with Lightroom.
Personally, I’m pretty comfortable with XMP and my ability to use them to carry along the changes I have made to the images. When I migrate images out of my main catalog to my backup storage array, I copy them at the OS level and copy the original raw file and the XMP file that has the metadata and development changes. I can then reimport those images in to another catalog (or another tool that can read XMP) and have my changes reflected.
As someone noted in the forum, though, that’s fine as long as you know that is not included in the XMP sidecar file so that you can make an informed decision, so I want to put together a quick post on the topic.
What Are XMP Files
XMP sidecar files are XML files that go along with your raw files that reflect modifications that you have made to an image. Remember that Lightroom does what is known as non-destructive editing. That means it doesn’t change your original raw file, but it has to keep track of all your changes somewhere. For the most part, all of that gets stored in your Lightroom catalog, but XMP provides a way to externalize those changes outside of the catalog so they can be read by other tools, including Lightroom and Photoshop.
You can have Lightroom automatically write to and update an XMP file for every change that you make to an image, or you can manually create and update an XMP file by using the Save Metadata to File… option in Lightroom. Your XMP files will be stored in the same directory as your raw image file, and have the same file name with an .xmp extension.
What Is Not Stored In XMP
There are some key items that are only stored in the Lightroom catalog. These items can be copied between catalogs by using the Import from Another Catalog and Export as Catalog options, but they are not stored in the XMP sidecar file. If any of these elements are important to you and your workflow, you might be better off using the catalog options instead of relying on XMP.
Your edit history on an image is not retained. The XMP file stores the metadata and develop module settings for the current state of the image when the file is written. No edit history means no undo and no revert.
Virtual copies and the changes made to virtual copies are not stored in the XMP. If you branch an image with a virtual copy and save the metadata changes, the XMP file will contain the state of the edits made to the original image, not the virtual copy. That means that if your finished image is a virtual copy, those changes are only reflected in the Lightroom catalog.
While the star ratings are reflected in the XMP file, the pick flag (Flag as Pick / Set as Rejected) is not.
Collections and Stacks
Collections and other groupings of images (stacks) are catalog-isms, so they do not go along with the images in the XMP.
If you’ve made an adjustment but disabled it inside of Lightroom using the toggle switch, the XMP will not include information about the adjustment.
What Is Stored In XMP
EXIF / Copyright / Keywords
Since Lightroom doesn’t change your raw file, any of the metadata that is in those raw files when you made the image will still be in those raw files. The XMP files include some overlap of those attributes and also reflect changes that you made to many of those original raw file attributes. This includes things like the capture time, copyright data, and keywords. If you have a metadata preset, all of those attributes should be stored in the XMP, as well.
Develop Module Changes
All of the adjustments that you make to an image in the develop module are stored in the XMP, with the exception noted above regarding when an adjustment is disabled. That includes the value of all the sliders and the position of all of your adjustment brushes.
I found this one interesting. If you create snapshots inside of Lightroom, those snapshots and the development settings at each snapshot are stored, as well.
I’ve highlighted some of the key elements that aren’t and are stored in the XMP sidecar files. In my workflow, the attributes and adjustments that I care about are stored in the XMP files, so I’m comfortable relying on them as a record of my final images in the event that I need to import them back in to Lightroom or Photoshop. Your mileage may vary.
With all of this advice, the last tidbit I will pass along is a reminder to back up your catalog before you remove anything or make any large changes. If something unexpected does happen, you can revert to a previous version of your catalog and adjust your path accordingly.