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Preview Size

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New Member
Dec 24, 2015
Lightroom Experience
Lightroom Version
Lightroom Version Number
Lightroom Classic version: 9.3
Operating System
  1. macOS 10.15 Catalina
Hi all,
In Lightroom Classic, running Catalina, on a MacBook Pro, I have 36,542 images. Is it normal to have 34G of previews?
See screenshot.
Screen Shot 2020-08-10 at 11.14.07 AM.png
What happens if I delete the previews? I assume they will rebuild as I open images again.
It's really lethargic as you browse around, but if 30,000 of those are ones you rarely look at, you can delete them all, and set a rebuild standard preview running overnight for the reset (e.g. if you can identify by date or whatever).

The question is more about why you care. If you are worried they are slowing things down by being so numerous, they do not. If you are running short of disk space, you can do this, but it becomes a bit of a hassle as eventually they start coming back and you have to repeat at ever increasing intervals as you get more and more tight on space.

But you absolutely can. Smart previews are a bit different; I don't use them so will let someone else comment there. But previews you can just delete and they rebuild (or you can rebuild them).
I'm new at Lightroom Classic but I originally had over 200GB of previews. I had imported only JPEGs. The computer ran over 18 hours non-stop building these. I deleted them all and changed the Catalog Settings > File Handling Standard Preview Size to 1024 pixels (If I recall, it had been something over 6,000). I left the quality at Medium.

Now the previews folder is only 11 GB (but it says 21,000 files and my total JPEGs is over 70,000). Apparently Lightroom may build the previews on the fly the first time they are viewed. The previews now build almost instantly and even if they grow to over 40GB, that is a small fraction of how they started.

This is my observation so if I'm off base, I'm sure someone will explain.
The recommended setting is "auto", as it will look at your (then current) monitor settings and build accordingly (I think but have not tested to build to "fit" to that monitor). Building smaller saves space, but may cause it to rebuild on the fly if you use a larger preview, much as it does for zooming to 1:1.

So indirectly, having a larger monitor increases the space you commit to previews.
I work on a MacBook Pro, so sometimes I throw out the previews to free up space if I just finished working across hundreds of images from the same set, like from a two-week trip. Why? Because if I'm moving on from that project and culled them down to the top 50 or so, I'm storing hundreds of previews for images I won't look at again for a while. By throwing out the previews, Lightroom Classic will rebuild and store previews only for the completely different, and usually much smaller, set of images I’ll browse through over the next week, and I won't miss the deleted previews that are for the hundreds of images I'm done with for now.

Also, two more points:

If Generate Previews in Parallel is on in Lightroom Preferences > Performance, Lightroom Classic will use unused CPU cores to build them in the background.

So indirectly, having a larger monitor increases the space you commit to previews.
Yes, and the big surprise for some is how big the Standard Preview Size (Catalog Settings > File Handling) can be for a “small” display. For example, on my 13" MacBook Pro, Lightroom thinks that Standard Preview Size: Auto should be 3360 pixels across. But for my 27" desktop display, it thinks Standard Preview Size: Auto is just 2560 px. Why?

Because the MacBook Pro has a Retina (or HiDPI in Windows) display. The 13" Retina laptop display has more pixels than the conventional 27" display. So logically, but unexpectedly for many, the file size for Auto previews on the little laptop is much larger than for the big desktop display.

If you use a Retina/HiDPI display and you think Lightroom Classic previews take up too much space, you can check how your Standard Preview Size is set, and decide whether you’re OK with lowering the Standard Preview Size to save some space.
Conrad, that makes a lot of sense. I've got photos I may not preview again for quite some time.
Thanks for the reply...
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