1:1 previews
1:1 previews are full-resolution previews Lightroom Classic builds so you can zoom to 100% without delay. They're stored in a separate preview folder, separate from your photos and catalog. Especially handy when judging sharpness while culling.
If you zoom in a lot to check sharpness and detail, 1:1 previews stop Lightroom from rendering each time. Rendering does take time and space, so you can build them on import or only when you need them. You can have old 1:1 previews discarded periodically to free up disk space. Tip: build them overnight for big shoots, then you'll cull smoothly at 100% the next day.
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The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.