A reader named Sean posted a comment asking why after deleting his 1:1 previews that the size of his preview file didn’t shrink. I found a number of other people having the same issue, and many people suspected that it was a bug in Lightroom or something that only affected certain systems. But then I stumbled upon a link with a possible explanation:
1:1 previews will not be deleted unless your chosen standard-sized preview is less than half the size of the full-sized preview.
With all the people with different cameras shooting in different modes with different resolutions using different standard preview sizes, it seemed like a reasonable explanation to why the behavior seemed so random. But being an engineer, I had to run the experiment!
Lightroom allows you to select from 5 different preview sizes: 2880 pixels, 2048 pixels, 1680 pixels, 1440 pixels, and 1024 pixels. General guidance is to set the value to the closest match for the resolution of your primary display to avoid having Lightroom generate unnecessarily large previews for no reason.
If you have a large screen, though, and use a large value for the standard preview, or if you are using an older camera with a lower resolution, the combination could result in a situation where discarding your 1:1 previews doesn’t actually delete the previews.
I wanted to experiment with my working catalog, so I started off by deleting my preview file. Next, I opened Lightroom and generated 1:1 previews for my 1,725 original, raw images. That resulted in a preview file size of 4.85 GB.
Next, I set my standard preview size to 2,880 pixels, which is greater than 1/2 the standard preview size for all of my cameras that have images in my catalog, which I’ve listed below. That means that discarding 1:1 previews with a standard preview size of 2,880 should not reduce the size of my preview catalog. To change the standard preview size, go in to your catalog settings by navigating to Lightroom > Catalog Settings and going to the File Handling tab.
|Delete 1:1 Previews With Standard Preview Of|
|Camera||Resolution||1/2 Full-sized Preview||2880||2048||1680||1440||1024|
|Nikon D7000||4,928 × 3,264||2464 pixels||N||Y||Y||Y||Y|
|Nikon D700||4,256 × 2,832||2128 pixels||N||Y||Y||Y||Y|
|Nikon D70||3,008 × 2,000||1504 pixels||N||N||N||Y||Y|
Indeed, when I tried to discard my 1:1 previews by navigating to Library > Previews > Discard 1:1 Preview, the catalog size remained 4.85 GB and the process of discarding the previews only took a few seconds (most likely because it didn’t actually do anything).
Next, I dropped my standard preview size down to 2048. Based on some very complicated math, I expected the previews for both the D7000 and D700 would be removed, and since those images constituted the bulk of the images in the catalog, the preview file size should reduce significantly. This time, the discard process actually took a few minutes to complete and, post-discard, the preview file size shrank to 1.67 GB.
Finally, the previews for my D70 will only be removed if my standard preview is 1440 or below. I first set the standard preview to 1680 and discarded the 1:1 previews, and the catalog size did not change since I had already deleted the previews for the D7000 and D700, and the 1680 standard preview resolution was greater than the 1/2 full-sized resolution of the D70. When I set the standard preview to 1440, sure enough, the preview file size shrank to 1.33 GB.
Based on the experiment, It seems that the standard preview size explanation is valid. Sean also let me know that when he changed his standard preview size that his catalog shrank by 30 GB after he restarted Lightroom. Good news!
Hopefully, if you’re here because you are wondering why your preview file size is so big that this post helped explained why discarding the 1:1 previews didn’t fix your problem!