Lightroom Preview size and quality

Question

A question came up recently in a discussion about Lightroom with Zoom (long, boring and not relevant here).

What happens if you have a 1920x1080 display, but make your Previews during import 2880. I thought it wasted some space - but nothing else would happen.

So, why DO we size previews as "as close as possible to the long dimension of the display, but not smaller"? Does it slow Lightroom because it has to match a 2880 px preview to a 1920 display rather than having to match a 2048 preview to the display?

I displayed the standard preview, but didn't see any difference it time to display. I didn't magnify any of them to 300 or 400% to see how they held up, though.

The discussion then morphed into - why aren't we using the highest QUALITY previews possible? Why does everybody use Low or Medium? Some, I believe, use Low because they've read something on the Internet that said using Low quality previews may improve Lightroom performance.

But, why DON'T we use High quality previews?

I did a little testing, which may be COMPLETELY inaccurate since I'm not sure what I'm doing. I imported the same image at different sizes and qualities. The 1440 px, Medium quality preview is about 1/3 the size (6.9 MB) of the 1440 px, High quality one (17.2MB). And the 2880 High quality preview is a couple MB larger than the 2880 px, Medium quality one (22MB).

But in the Loupe View I didn't see any perceptible differences - but I wasn't editing and making dozens of changes. As it gets magnified, does the Medium quality preview degrade sooner? At 100%? 200%?

Can someone explain a little about what happens inside Lightroom with respect to how previews work? Does a Low quality preview look or act different than a High quality preview? Does it pixellate or degrade as the processing steps are applied?

Is there an advantage to creating High quality previews versus Medium? Disavantage other than a couple MB of preview size?

DavePinMinn · Accepted Answer

Thanks Rob. That was one of the articles I went through looking for actual empirical data on the impact of preview size and quality. I still haven't found any. They said:

"For Standard Preview Size, choose the amount that is closest to the longest edge of your screen resolution, but not shorter than it. For example, if your screen resolution is 1920 x 1200 pixels, choose Standard Preview Size > 2048 Pixels."

And for quality:

"For Preview Quality, choose Low or Medium, which equates to the low- or mid-range of the quality scale for JPEG files."

Though not ancient, the article thought a 2560x1600 display was high-res. These days people routinely use 3840x2160 and some even 5120x2160 or 2880 displays. Are the low or mid-range quality previews the best choice when trying to determine how sharp an image is on these larger screens? Or, does the larger preview size, even at medium quality exceed what the displays can show?

Rob_Cullen · Answer

There is some suggestions in this 'Help' article-

https://helpx.adobe.com/lightroom-classic/kb/optimize-performance-lightroom.html

"Keep standard size previews as small as possible..."

Apparently, "Quality" is a JPG compression scale.

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