Sharpening does not recover detail
It is worth being blunt about this before you start, because the expectation is what makes people overdo it.
Detail that was never recorded is not in the file. If the focus missed, or the camera moved during the exposure, the information about what those edges looked like does not exist and nothing can put it back. Every tool promising to fix a blurry photo is either making things up or quietly doing what this does.
What sharpening actually does is increase contrast along edges that are already present. The eye reads high edge contrast as sharpness, so the picture looks crisper without a single new piece of detail being added.
On a slightly soft photo that is a genuine and substantial improvement. On a properly blurred one it produces a blurry photo with halos.
How an unsharp mask works
The name comes from a darkroom process and describes the middle step rather than the result, which is why it confuses everybody.
Blur a copy of the image. Subtract that copy from the original. What is left is the difference between them, which is precisely the fine detail the blur removed: the edges. Add that difference back onto the original, amplified, and every edge gets pushed further apart than it was.
That is all of it, and it is what Photoshop, Lightroom and every other editor are doing behind a slider labelled sharpen.
The three controls, and which one to move first
Radius is how wide the detail being sharpened is. Small values, one or two pixels, target fine detail like text, eyelashes and foliage. Larger values sharpen broader transitions and go wrong quickly, because a wide radius is what produces visible halos. Start at one or two and leave it there unless you have a reason.
Amount is how hard the edges are pushed. This is the one to adjust. Somewhere between 60 and 120 percent suits most photographs; past about 150 you are usually making a decision rather than a correction.
Threshold is the one people ignore and the reason their sharpening looks bad. It tells the process to skip differences smaller than a set amount. Without it, grain and sensor noise are edges too, and they get sharpened with the same enthusiasm as the subject. Raise it until flat areas like sky stop crawling.
Halos are how everybody knows
Sharpening lightens one side of an edge and darkens the other. Push it far enough and that becomes a visible bright outline running around every object in the picture.
Once you have noticed halos you cannot stop noticing them, and they are the clearest signature of an over-processed image. They are also considerably more obvious in print than on a screen, and it is why photographs that looked fine on a laptop come back from the printer looking crunchy.
If you can see a light rim against the sky, the amount is too high or the radius is too wide. Reduce the radius first.
A shrunk preview is a lie
This is the reason this tool looks different to the others on the site.
Scaling a photograph down averages neighbouring pixels together. Averaging hides halos, hides noise, and hides the crunchiness that sharpening introduces. A preview at 25% will look excellent at settings that are visibly wrong at full size, every time.
So the preview here shows actual pixels at 100%, and you can click anywhere to move the view. Judge it on an edge that matters, usually the eyes on a portrait or the nearest text on a document, and ignore how it looks small.
Sharpen last
Sharpening belongs at the end of the process, after resizing and after any tonal work.
Resizing re-samples the image and softens it, so anything you sharpened beforehand gets partly undone and you end up sharpening twice. Lifting exposure or contrast afterwards changes the edge contrast you just carefully set. Do everything else, resize to the size it is actually going to be used at, then sharpen, then export once.
And keep the original, because sharpening is not reversible from the saved file.
Common questions
Can sharpening fix a blurry photo?
No, and anything claiming otherwise is selling something. Detail that was never recorded is not in the file, and no amount of processing invents it. What sharpening does is increase the contrast along edges that are already there, which the eye reads as sharper. A slightly soft photo improves a lot. A genuinely out of focus or motion blurred one cannot be rescued.
What is an unsharp mask?
The standard sharpening method, named after a darkroom technique and used by every editor including Photoshop. It makes a blurred copy of the image, subtracts it from the original to find where the edges are, then pushes those edges further apart. The name is confusing and describes the blurred copy, not the result.
Why does my sharpened photo have white lines around things?
Those are halos and they mean too much amount, too large a radius, or both. Sharpening works by making one side of an edge lighter and the other darker, so pushed too far it produces a visible bright outline. Halos are the single clearest sign that an image has been over-processed, and they are much more obvious in print than on screen.
What does the threshold do?
It tells the sharpening to ignore differences smaller than a set amount, which is what stops it sharpening noise. Without a threshold, film grain and sensor speckle are edges too and get enhanced just as enthusiastically as the eyelashes. On a clean photo leave it low. On anything shot at high ISO, raise it until the flat areas stop crawling.
Why does the preview show actual pixels rather than the whole photo?
Because sharpening cannot be judged on a shrunk image. Scaling a photo down averages pixels together, which hides halos and hides noise, so anything that looks right in a small preview is usually overdone at full size. This shows you a 100% crop, and you can click to move it, because that is the only view that tells you the truth.
Should I sharpen before or after resizing?
After, always. Resizing an image re-samples it and softens the result, so sharpening first means you sharpen detail that is then thrown away and softened again. Resize to the size you are actually going to use, then sharpen, then save once.