Lens correction
Lens correction fixes the optical flaws your lens introduces: distortion, dark corners (vignetting) and color fringing. Based on a lens profile, the software straightens this automatically. One checkbox and your photo instantly looks cleaner and straighter.
Turn on 'Enable Profile Corrections' in the lens correction panel, and Lightroom recognizes your lens and applies the right fixes. With wide-angle lenses the bulge disappears, and on many lenses the corners brighten. Note: vignette correction can bring up noise in the corners, so check at high ISO. Do lens correction early in your workflow, because it shifts the image edges slightly and that affects your crop.
Read also the in-depth explanation: Lens correction explained.
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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.