Importing a photo into Lightroom is a 2 pass process:

  1. Create all the database entries to keep track of the photo and convert it if necessary
  2. Create the previews

Lightroom has 2 set of previews the standard previews and the 1:1 previews. All these previews are stored in the thumbnails-cache.db database located in the xyz Previews.lrdata folder/directory with xyz is the name of your catalog. Today, we will discuss the standard previews. The settings are per catalog, with Edit > Catalog Settings > File Handling tab.

The standard previews are used:

  1. At the bottom of the screen, in the film strip
  2. At the centre of the screen, in the grid view and the survey mode

Lightroom gives you the choice of the size of the standard previews:

  • 1024 pixels
  • 1440 pixels
  • 1680 pixels
  • 2048 pixels

So which one to use and what are the implications. The correct answer is: 1½ times your screen resolution. My screen resolution is: 1656 pixels wide by 856 pixels tall. So 1656 x 1.5 = 2484. The closest is 2048. That’s what I use.

  1. It does not really matter if you set the “wrong” standard preview size. Lightroom will generate the correct preview size on the fly, but it will take extra time for changing the preview size.
  2. The thumbnails-cache.db database located in the xyz Previews.lrdata folder/directory can be deleted. Lightroom will re-generate it as it needs it with the required standard previews. That’s the way I change the catalog setting of the standard previews.
  3. The setting is for the whole catalog.
  4. The size of the thumbnails-cache.db database will be 4 times smaller at 1024 pixels than at 2048 pixels. So it’s a balance between speed vs. space.
  5. Today, storage is cheap…