Image Sharpen Tool
Sharpen an image with an unsharp mask or a convolution kernel, with radius, amount and threshold controls, and a halo warning when the settings are pushed too far.
Last reviewed by the Radiatus Cloud team
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Sharpening cannot recover detail
No sharpening operation adds information. It increases the contrast at edges, which makes the boundaries between areas more distinct and reads to the eye as sharper. A genuinely out of focus photograph has lost the detail and sharpening only makes the blur more contrasty. What sharpening does fix is the slight softness introduced by a camera sensor's anti-aliasing filter, by demosaicing, and by resizing, all of which are real and all of which are recoverable.
Unsharp mask is subtraction, not addition
The name is confusing and describes the process exactly: blur a copy of the image, subtract it from the original to find what the blur removed, and add that difference back at some strength. What the blur removed is the fine detail, so adding it back amplifies exactly that. The technique predates computers, coming from darkroom practice where a blurred negative was sandwiched with the original.
Three controls, and the threshold is the one people skip
Amount sets how much of the difference is added back, radius sets how wide the blur is and therefore what counts as detail, and threshold sets how much difference a pixel needs before it is sharpened at all. The threshold is what stops sharpening from amplifying noise in flat areas such as skies, where there is no detail to sharpen and every variation is grain. Skipping it is why over-sharpened photographs have visibly noisy skies.
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Frequently Asked Questions
Can sharpening fix an out of focus photograph?
No. The detail is gone and sharpening only increases contrast at whatever edges remain, making the blur more contrasty. It does fix the slight softness from sensor filters, demosaicing and resizing, which is real softening of detail that is still present.
What does the radius control?
How wide the blur used for comparison is, which determines what counts as detail. A small radius sharpens fine texture, a large one increases broad local contrast and starts to look like a contrast adjustment rather than sharpening.
What is the threshold for?
It stops pixels being sharpened unless they differ from their surroundings by more than the set amount, which keeps noise in flat areas such as skies from being amplified. Skipping it is why over-sharpened photographs have grainy skies.
What are halos?
Bright and dark outlines that appear along high-contrast edges when the amount or radius is too high. They are the sharpening itself becoming visible, and once present they cannot be removed by further adjustment.
When should I sharpen?
Last, after resizing, and to suit the output. An image sharpened for print looks over-sharpened on screen, because print softens slightly and a screen does not. Sharpening before resizing means the resize then softens what you just did.
Privacy & Security
Everything runs in your browser; nothing is uploaded.
How to Use
Upload an image and adjust the sharpening amount, radius and threshold.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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