A smart preview in Lightroom Classic is a compressed, lossy DNG copy of your raw file, 2560 pixels on its long edge, stored next to your catalog. Its point is that you can edit with it: when the external drive holding your originals is unplugged, the Develop module keeps working, and the edits apply to the originals as soon as the drive is back. Standard and 1:1 previews look similar on screen but are rendered images for viewing only. You cannot develop from them.
So the choice is not which preview is best. Standard or embedded previews make browsing fast, 1:1 previews make zoomed focus checks fast, and smart previews make you independent of the drive. Most photographers want the first two on every shoot and smart previews only on the shoots they will edit somewhere else.
What is the difference between smart, standard and 1:1 previews?
Lightroom Classic keeps two separate stores beside the catalog file:
- Previews.lrdata holds the viewing previews. Minimal and Embedded & Sidecar use the small JPEG your camera already wrote inside the raw, so they appear almost instantly but do not show your edits. Standard previews are rendered by Lightroom at about the size of your screen. 1:1 previews are rendered at the full pixel size of the original, which is what you see when you zoom to 100 percent in the Library module.
- Smart Previews.lrdata (prefixed with your catalog's name) holds the smart previews: real raw data, cut down to 2560 pixels on the long edge and lossy compressed, often around a megabyte each against 25 to 60MB for a modern raw.
That last line is why smart previews exist. A wedding of 3,000 frames is a small folder as smart previews and a large one as raws.
Which previews should you build at import?
In the Import dialog, the File Handling panel on the right has a Build Previews menu and a separate Build Smart Previews checkbox. A practical rule:
- Embedded & Sidecar when you cull straight after import. It is the fastest way to start flagging, because frames appear without a wait.
- Standard when you will browse and compare in the Library grid before editing, and can let the import run while you do something else.
- 1:1 for work where you check focus on every frame, such as ceremonies, sport and events. It is slow to build and heavy on disk, so start it and walk away.
- Build Smart Previews only if you will edit with the originals offline, or if your machine struggles in Develop (see the preference below).
Previews can always be built later from Library > Previews, which has Build Standard-Sized Previews, Build 1:1 Previews and Build Smart Previews. Select the photos first, or it works on the whole folder or collection.
How do you edit in Lightroom without the external drive?
- With the drive connected, select the shoot and choose Library > Previews > Build Smart Previews. Wait for the progress bar in the top left to finish.
- Unplug the drive. The folder turns grey with a question mark in the Folders panel, which is expected.
- Edit as usual. The Histogram panel shows Smart Preview under the graph when you are working from one, Original when the raw is available, and Original + Smart Preview when both are.
- Reconnect the drive later. Lightroom reads the edits from the catalog, so there is nothing to sync or copy.
This only works if the catalog itself lives on the laptop's internal disk. If the catalog is on the external drive too, unplugging it takes everything with it. The catalog organisation post covers where to keep it.
When the editing happens on a different computer, use File > Export as Catalog, untick Export negative files, and tick Build / Include Smart Previews. The exported catalog fits on a USB stick. Back home, bring the edits in with File > Import from Another Catalog. It is the same arrangement used when outsourcing editing, and the raws never leave your desk.
Can you export from a smart preview?
Yes, with the original offline, at up to 2560 pixels on the long edge, because that is all the data there is. That is plenty for social media, a website, a blog post or a set of sneak peeks sent the evening after a wedding. It is not enough for a print longer than about 8.5 inches at 300 dpi, or for full resolution client delivery.
Check the Histogram label before exporting a client gallery. Lightroom does not stop you exporting from a smart preview, and with Don't Enlarge unticked it upsizes the 2560 pixel file to whatever size you asked for. That gallery looks fine on a laptop until the client orders a 20 x 30 inch canvas.
Should you turn on "Use Smart Previews instead of Originals"?
The checkbox is under Preferences > Performance (in the Lightroom Classic menu on a Mac, the Edit menu on Windows): Use Smart Previews instead of Originals for image editing. With it on, Develop works from the smart preview even when the raw is connected, which makes sliders and brushes noticeably quicker on an older laptop or with files from a high resolution body. Lightroom switches to the original when you zoom to 100 percent and when you export, so output quality is not affected.
The catch is that you need smart previews built for it to do anything, and you judge sharpening and noise reduction at fit to screen size on a slightly compressed file. Zoom to 100 percent before you settle those two. If your machine already edits smoothly, leave it off. If you are deciding whether a new laptop is worth it, try this first, and the laptop buying guide lists the specs that actually change editing speed.
How do you stop previews filling up your disk?
1:1 previews are the usual culprit. In Catalog Settings > File Handling (Lightroom Classic menu on a Mac, Edit menu on Windows), set Automatically Discard 1:1 Previews to After One Week or After 30 Days. Never keeps them forever, and on a busy wedding season that can grow to tens of gigabytes. In the same panel, leave Standard Preview Size on Auto unless you have a reason.
Smart previews are not discarded automatically. Once a shoot is delivered and archived, select it and choose Library > Previews > Discard Smart Previews. The edits stay in the catalog and the originals are untouched.
The export side follows the same rule: anything that goes through File > Export uses the originals when they are online and the smart previews when they are not. That includes an export plugin such as the Xposure one, which uploads straight into a client gallery, so plug the drive back in before you send a full resolution delivery. Tonight, open the Histogram on your current shoot, see which label it shows, and build smart previews for the next job you plan to edit away from your desk.
