About the Sharpen Image
Unsharp masking is the technique every photo editor uses, and the name is honest about how it works. A blurred copy of the picture is subtracted from the original to isolate the fine detail, and a multiple of that difference is added back. Edges gain local contrast, which the eye reads as sharpness, while flat areas are left alone because their difference from the blur is near zero.
Amount scales how much of the difference returns. Around eighty to one hundred and twenty percent suits a slightly soft photograph; going far past two hundred produces bright halos along high contrast boundaries and a crunchy, over processed look. Radius sets the size of the blur used to build the mask, and it should track the size of the detail you care about: one pixel for text and fine texture, three or four for a portrait where you want features to firm up without emphasising pores.
Threshold is the control most people skip and then miss. It refuses to sharpen a pixel whose difference from the blur is smaller than the number you set, which leaves smooth skies, skin and shadow noise untouched while edges above the limit are still enhanced. Set it to four or six on a noisy phone photo. No amount of sharpening can recover detail that a lens or a heavy compression pass never recorded; it makes existing edges more visible, nothing more. Save as PNG when the picture will be edited again, since a JPG pass adds its own artefacts along the edges you have just emphasised. For softening rather than sharpening, Pixelate Image and Censor Image go the other way.
How to use
- Drop a slightly soft photo or scan onto the left pane.
- Start with Amount at 100 and Radius at 1, then raise the amount slowly.
- Increase Threshold to four or six if grain in flat areas starts to sparkle.
- Compare against the original before downloading, and back off if halos appear.
Common questions
- Can sharpening fix a badly out of focus photo?
- No. It raises the contrast of edges that are already recorded. Detail that the lens never captured cannot be created by any amount of processing.
- What causes bright outlines around objects?
- That is a halo from too much amount or too large a radius. Lower the amount first, then reduce the radius to one or two pixels.
- What is the threshold control for?
- It sets the smallest difference worth sharpening, so gentle gradients such as sky and skin stay smooth while real edges are still enhanced.
- Should I sharpen before or after resizing?
- After. Resizing softens a picture, so sharpening last matches the amount to the final pixel dimensions.