How image sharpening works and how to avoid halos
The edge-contrast math behind sharpening, why it cannot truly unblur a photo, and how to pick an amount that looks crisp instead of crunchy.
Sharpening is about local contrast, not new detail
A sharpen filter does not add information that the camera never captured. It finds places where brightness changes, the edges in a photo, and increases the difference across them. Pixels on the light side of an edge get a little lighter and pixels on the dark side get a little darker, which makes the boundary read as crisper to the eye. Because it works only with the detail already present, sharpening enhances a soft image but cannot reconstruct one that was genuinely out of focus.
The 3x3 kernel this tool uses
This tool applies a small convolution to every pixel. The center pixel is multiplied by one plus four times the strength, and each of its four orthogonal neighbors is subtracted at the strength value. In a flat area where the neighbors equal the center, those subtractions exactly cancel the boost, so smooth regions such as skin or sky stay clean. Only near an edge, where neighbors differ from the center, does the math push the values apart and create the crisper look. Every pixel is read from a snapshot of the original, so the filter never feeds partly sharpened values back into itself.
Why halos and noise appear at high amounts
Push the amount up and the contrast boost across an edge grows until it overshoots, leaving a bright rim on one side and a dark rim on the other. Those rims are halos, and they are the classic sign of oversharpening. The same amplification also lifts fine noise and JPEG compression blocks that were barely visible before, giving flat areas a gritty texture. Strong sharpening is useful for flat scans and technical images, but for photos it quickly crosses from crisp into harsh.
Picking an amount that looks natural
Start with a preset that matches your goal: Subtle for portraits and anything where texture should stay soft, Medium for general photos, Strong for flat or low-contrast source material. Then nudge the Amount slider while watching the preview at full size, stopping just before halos appear along high-contrast edges. If the image already has visible noise, sharpen less, since the filter will make that noise more obvious. When in doubt, a lighter touch almost always ages better than an aggressive one.