Lightroom is first and foremost a virtual photo organizer and darkroom. The virtual aspect is what makes Lightroom’s non-destructive workflow possible, but it also means the catalog system can be confusing. One commonly heard refrain is, “Where are my picture files?” Fortunately, most users discover that importing pictures into Lightroom’s Catalog is not the same as storing the actual files or data bits from those pictures, inside the Catalog. If it did, your Catalog would quickly become so enormous that it would take a long time just to launch the application!
The Lightroom Catalog is a special kind of database that stores only references to your imported pictures. It knows where they are and what they look like, and the kind of camera, lens, and exposure settings that created them.
Thus, in a raw workflow, Lightroom does not modify the data inside your original photos, nor store modified copies of those photos. Instead, a behind-the-scenes system of metadata and high-quality previews are used to facilitate the changes you make. This non-destructive workflow means you can easily pick up where you left off—or start over at any time—without loss of original image quality.
However, for a time, there were limitations with this virtual setup, especially if your imported picture folders were located on an external hard drive(s), and you had to disconnect for a time. When that happened, Lightroom would remember the original location of your files but not be able to “see” them. This limited the number and type of operations you could perform on those pictures while offline.
You could still apply Library attributes (labels, ratings, etc.) and other organizational processes, but you could not edit imported photos from those disconnected drives in the Develop module. All that has changed with Lightroom 5. The new Smart Preview feature allows you to organize, develop, and even export images, whether you’re connected to your main photo archive at the office or working at the airport on a laptop. As long as your catalog is with you and you generated Smart Previews before you left the office, you can take full advantage!
Smart Preview Characteristics
Before you create Smart Previews, it’s useful to understand a few things about the preview files themselves. Instead of using a JPEG file, Lightroom 5 uses the Lossy DNG format to create Smart Previews that are “lightweight” (i.e., compressed) but that maintain their color and detail more faithfully than a JPEG compressed by a similar amount or at a similar file size.
These special preview files reside in the same folder structure as the Lightroom catalog (my advice is to leave them right where they are!), similar to the standard previews. At the time of this writing, Smart Previews were limited to 2048 pixels on the long dimension, meaning that when you zoom to 100%, you will not see the same level of detail as with a full-sized standard preview (especially 1:1 previews). The exception would be if you set the camera to capture the original files at a very small size that fit within the bounds of the 2048-pixel limitation.
Keep in mind that when both the standard file preview and a Smart Preview are present, Lightroom will always default to working with the (larger) standard preview, providing the most detail available. When the Catalog is not connected to the drive where your imported photos are located, it will then switch over to the Smart Preview. Smart Previews can be generated either from scratch during import or from another catalog.