Today’s Question: I have an SSD that houses much of my photo collection in the Lightroom Classic catalog (approx. 44,000 images). This drive is not always plugged in to my laptop; rather I plug it in weekly to make sure it is backed up to Backblaze. When the SSD is not plugged in and I want to look at any of the older files in the catalog, many of the previews do not show up [appearing as blank gray thumbnails]. What is your recommended preview format in Lightroom Classic? Is it possible to redo the previews after photos have been imported so all previews are of the same format?
Tim’s Quick Answer: I generally recommend generating Standard previews in Lightroom Classic, though in this particular case you may want to opt for the 1:1 (full resolution) preview option. You can generate those previews at any time using the commands found on the Library > Previews menu.
More Detail: Once your photos are imported into your Lightroom Classic catalog there are two options for previews in the context of the Library module. Those options are Standard previews and 1:1 previews.
The 1:1 previews are generated at the full resolution of the source images. For Standard previews you can adjust the size and quality settings on the File Handling tab of the Catalog Settings dialog. I generally recommend using the Auto option for Standard Preview Size and the Medium option for Preview Quality. These settings provide a good balance between image quality and the amount of hard drive space consumed by the previews.
To generate the previews at any time you can select a group of photos and then go to the menu and choose Library > Previews followed by either “Build Standard-Sized Previews” or “Build 1:1 Previews”.
The Standard previews are roughly the resolution of your monitor display, so they work well for general browsing. If you want to be able to zoom in on the photos when the hard drive is disconnected, you will want to generate the 1:1 previews instead. Otherwise, for images that only have a Standard preview, zooming in will result in a preview of reduced quality due to relatively low resolution.
In the Develop module the preview for each image is generated on the fly based on the source image file and the adjustments you have applied. This is therefore the best way to review images in terms of optimal image quality. It is possible to generate Smart Previews for the images, so that you can work in the Develop module even when the hard drive is disconnected. However, this can consume considerable space on the internal hard drive, so unless there’s a compelling reason to generate Smart Previews I recommend simply connecting the hard drive when you want to work in the Develop module.