First we have the Previews.lrdata file. This is usually named to match the Catalog name, but with ' Previews' added. For example if I had a Catalog called 'BigFluff.lrcat', then the associated previews would be called 'BigFluff Previews.lrdata'. While I've referred to it as a file, on Mac it's really a package, which is actually a folder that has access blocked. On PC, it's just a folder.
If we look inside the Previews (on Mac, Ctrl Click and choose Show Package Contents, on PC, just open the folder), we generally see a 'previews.db' file, a 'root-pixels.db' file and folders. The '.db' files are the same database as the Catalog, which are SQLite files. Not essential knowledge, but you can get SQLite tools to look inside these files if you like. Our example catalog actually only has one image in it, but looking at the 'pixels.db' file, you can see quite a few entries, which also show a number of available dimensions.
The folders themselves contain the previews, which are custom Jpeg files. Generally you'll more than one preview per photo, to cover thumbnails and standard or 1:1 previews, sometimes even more. Going by our pixels.db file, we have 8 different dimensions in our previews.
Going back to our Lightroom Catalog folder, I'll quickly mention the other files. The previews.lrcat file is the Catalog, which I called 'previews'. The other files (ending in -journal and .lock) are only visible when Lightroom has the catalog open. The journal file records updates to be written to the catalog, while the .lock file prevents others opening the catalog to prevent corruption.
Now that we've looked at the anatomy of Lightroom previews, let's look inside the program to see how we can affect what Previews Lightroom creates. The first place we generally see a reference to Previews is in Import. In the File Handling panel, on the top right, we have the Render Previews drop down menu. In it are 4 options. Minimal, which Lightroom makes only a thumbnail. Embedded & Sidecar, where Lightroom uses the internal previews generated by the camera. Standard, which is a medium sized preview, the size and quality of which is set in Catalog Settings. Finally, there is 1:1, which creates a full size preview.
Once inside Lightroom, the next place we see mention of Previews is in the Library>Previews Menu. Here you can render Standard or 1:1 Previews, or Discard 1:1 Previews. Discarding is not instant, just in case you need to undo, but will happen after a time. When you choose the render command, Lightroom does a quick check to see if a preview of that size exists, and then renders files that have the chosen size missing.
Finally, we should look at the Catalog Settings for Previews, as mentioned earlier. Catalog settings are in the Lightroom Menu on Mac, and the Edit Menu on PC. When the dialog appears, click on the File Handling tab. The top section is Preview Cache, which is what we're looking for. Standard Preview size is set in the first dropdown. Set this to be slightly larger than your screen size. I use 2048 on a 1900 pixel wide monitor. 30" monitor users might find this too small for Lightroom, and that it generates 1:1 previews instead. Quality sets the compression level of the previews. The default is medium, but you can set it higher or lower to either get a better preview, or save disk space. I should mention the previews themselves are stored in Adobe RGB colour. The final option is the discard time for 1:1 previews. Generally if you don't go back to images after a week, set it to one week, or to 30 days if you need longer. Whatever you set, Lightroom will create a 1:1 preview as it needs it. You might be okay with the time this takes, and can set a shorter discard time to save disk space.
One final comment on Previews is that no matter what option you picked on Import, Lightroom will create previews as and when it needs them. If you deleted the entire .lrdata package/folder, Lightroom would create a new one and start generating previews from scratch.
© Sean McCormack