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When oversharpening an image (w/o protecting detail) in Photoshop a distinct pattern emerges -is there a reason for this? does it maybe have to do with how the tool was coded?

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up vote 6 down vote accepted

The slightly unintuitive name for what "sharpen" does is an "unsharp mask". There's a good explanation of the process here, but roughly the idea is to make a blurred copy of the image (the "unsharp" part), and compare it to the original.

There will be large differences near edges, and less of a difference where the image was already smooth. The magnitude of the difference is then used as a mask when increasing the contrast of certain areas, meaning edges get more contrast applied.

If you're referring to the maze-like pattern in your image, I think that's because sharpening repeatedly will start to over-highlight natural noise in the image which is close to the same frequency as the radius of the unsharp blur. Presumably the default blur is about a pixel or two in radius, so it's exposing the subtle one or two pixel wide ripples of noise which were there in the original.

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