Today’s Question: The question about the Previews file [in Lightroom Classic] brings up another: Is there any reason to keep a growing Previews file? Can you periodically delete it and start fresh?
Tim’s Quick Answer: The primary reason to keep the Previews file for Lightroom Classic is to optimize performance when browsing your photos in the Library module. If you want to recover space, or suspect the Previews file is causing other problems, it can be safely deleted and Lightroom will re-build previews as needed. Just be sure to only delete the Previews file, and not the actual catalog file.
More Detail: The Previews file for Lightroom Classic contains what are essentially JPEG image previews of your original captures being managed in your Lightroom catalog. In other words, that Previews file represents a cache for your images, enabling a faster browsing experience as well as providing the ability to view your images even when the original source files are unavailable. For example, if you store your original captures on an external hard drive, even with that drive disconnected you can still browse your photos based on the Previews that have been generated.
While the Previews file certainly provides benefits in terms of performance and offline browsing, there is no permanent harm caused by deleting the Previews file. If your hard drive is getting close to full, deleting the Preview file can provide a quick way to recover a considerable amount of free space. In addition, at times I’ve seen situations where some Previews get corrupted, leading to problems browsing images. Deleting the Previews file can resolve this type of issue.
The first step is to navigate to the folder that contains your Lightroom Classic catalog files. You can quickly get to that folder by clicking the “Show” button in the Information section of the General tab of the Catalog Settings dialog in Lightroom Classic. Within the folder that is revealed when you click this button, you can see the Lightroom catalog, which has a filename extension of “.lrcat”. That is the most important file in the folder containing your catalog files, as it is the actual catalog being used to manage your photo library. In addition to the lrcat file, you’ll see a variety of other files associated with the catalog. One of those will be the Previews file.
The Previews file will have the same base filename as your Lightroom Catalog, with “Previews” appended to that filename. The filename extension will be “.lrdata”. This file can be deleted, though I recommend first quitting Lightroom. After deleting the Previews file and launching Lightroom again, a new Previews file will be created automatically. Note that initially you won’t see any previews for your photos, but they will be generated as you browse your photos. That means the new Previews file will be considerably smaller than the file you deleted, but will gradually grow over time as you browse additional folders containing photos, and new previews are generated for the photos in those folders.