Lightroom uses numerous kinds of previews and caches for different purposes. Having a basic idea of their usage can help you pick the right ones for your needs. The main ones you need to know about are rendered previews and smart previews. To fully understand the Library & Develop preview loading logic to optimize your preview choices, see the diagrams on pages 503 & 506 of Adobe Lightroom Classic – The Missing FAQ. In this post, we’ll cover the basics for Previews and Cache.
Standard-sized rendered previews are the most frequently used previews. They’re used to display the photo in every module except Develop (where they’re shown briefly and then replaced with cached raw data). For speed, Lightroom stores a range of different size previews, from thumbnails right up to your chosen preview size.
The size and quality of the standard sized previews is set in the Catalog Settings dialog. Go to Edit menu (Windows) / Lightroom menu (Mac) > Catalog Settings > File Handling to check and change the settings.
The best size setting depends on your general browsing habits and on your screen resolution. Choosing a size about the width of your screen is a good starting point, and the default Auto setting does this automatically. If you always leave the panels open and your hard drive space is very limited, you may prefer a slightly smaller size.
The quality setting is, as with most things, a trade-off. Low quality previews take up less space on disc as they’re more compressed, but higher quality previews look better. The default, Medium, is a good compromise.
1:1 rendered previews are full resolution Adobe RGB JPEGs, so they take up more space. If you want to zoom in on your photos in the Library module (e.g. checking focus), it avoids Lightroom having to build 1:1 previews on the fly, which would slow your browsing experience. Note that they don’t have any effect in the Develop module, which works with raw data instead of pre-rendered previews.
If you’re concerned about the disc space that the 1:1 previews take up, you can choose to have them automatically deleted. You can select this to happen after 1 Day, 1 Week, 30 Days or Never. Go to Edit menu (Windows) / Lightroom menu (Mac) > Catalog Settings > File Handling to check and change this setting. Alternatively, you can discard 1:1 previews on demand by selecting the photos and choosing Library menu > Previews > Discard 1:1 Previews.
There are multiple ways to build standard or 1:1 previews:
- On Demand – The standard sized preview is automatically built when you view a photo in the Library module. The 1:1 preview is rendered when you zoom in. The problem with waiting for them to build on demand is you’re sat staring at a Loading overlay while Lightroom builds the preview. Of course, this slows down your browsing. To avoid this delay, you can build the previews in advance.
- On Import – In the File Handling panel of the Import dialog, you can choose to build Standard or 1:1 Previews immediately after import. The other options in the Preview pop-up – Minimal and Embedded & Sidecar – only extract previews embedded in the image file, leaving Lightroom to build its own previews on demand.
- Menu Command – If you select the photos in the Library module, you can select Library menu > Previews > Build Standard Sized Previews or Build 1:1 Previews. This is particularly valuable, as you can leave Lightroom building these previews at a time when you’re not using the computer. It can also be used to update existing rendered previews when you’ve made Develop changes. It automatically skips any previews that are already up to date.
Temporary Caches – Lightroom also uses a selection of temporary caches to improve performance, especially in the Develop module. The most useful of these caches is the full resolution pre-load, which happens automatically behind the scenes. When you’re working in the Develop module, it preloads some photos either side of your current photo. This means that when you move on, it loads much quicker. This happens automatically, so you don’t need to do anything to benefit.
Smart Previews are lower resolution (2560 px long edge) partially processed raw data, and can be used in place of the original files. They behave like the original raw files when editing in the Develop module, except they’re smaller, so they’re a lot less taxing on the computer’s processor. They also help to speed up face recognition and mobile sync.
To build smart previews, check the Build Smart Previews checkbox in the File Handling panel of the Import dialog, or after import, select the photos in the Library module and go to Library menu > Previews > Build Smart Previews. If you decide to delete them later, perhaps because you require the disk space, you’ll find Discard Smart Previews under the same menu.
To use them to improve Develop performance, simply go to Preferences > Performance and check Use Smart Previews instead of Originals for image editing. When you zoom into 1:1 or export the photos, Lightroom’s smart enough to switch to the full resolution original.
Camera Raw Cache (aka ACR Cache) temporarily holds partially processed fast load data for the most recently accessed raw files. It’s used in the Develop module to speed up loading times, although other optimizations have largely replaced it now.
You can set the size of the cache in Preferences > File Handling tab. The pixel dimensions of the cached images vary depending on your standard preview size. 5-10GB is plenty for most people, but if you have a high resolution monitor, you might want a bit more cache space.
Preview Area Size
Finally, the size of your preview in the Develop module makes a significant difference to the interactive performance. The lower resolution the preview, the fewer pixels Lightroom has to compute, and the quicker it runs. If you don’t mind a smaller preview, you have a few options. You can reduce the size of the Lightroom window, enlarge the panels to make the preview area smaller, or simply select a smaller zoom ratio (e.g. 25%) in the Navigator panel.
There are further tweaks you can make during your normal workflow to improve Lightroom’s performance. We’ll discuss these in the next post, workflow tweaks.
For extensive information on Lightroom Classic, see Adobe Lightroom Classic – The Missing FAQ.
If you have the Photography Plan, then as well as Classic you have access to the Lightroom cloud ecosystem including the mobile apps and web interface. For more information on these apps, see Adobe Lightroom – Edit Like a Pro.
Note: purchase of these books includes the first year’s Classic or cloud-based Premium Membership (depending on the book purchased), giving access to download the latest eBook (each time Adobe updates the software), email assistance for the applicable Lightroom version if you hit a problem, and other bonuses.
We also have a special bundle offer for the two books. This includes Premium Membership for the first year as described above for the whole Lightroom family!
Originally posted 10 October 2016, updated June 2021.