Sharpening: Why is background unaffected?

[tmyuen]

Re: Real-World Example in Tutorial "Sharpening: Unsharp Mask"

First off, I must confess that I've never used Unsharp Mask before :-) I just started to study the basics of it...

My understanding (may be wrong) is that when sharpening is applied to an image, the entire image will be affected. However, this doesn't seem to be the case when I look closely at the Real-World Example image (the old man) in the tutorial. The blurry background is not affected and only the subject is sharpened. Is this a unique phenomenon in Unsharp Mask? What about other sharpening algorithm?

[McQ]

The out of focus background does not appear to be sharpened (relative to the sharp foreground) because an unsharp mask effectively acts as a high-pass filter. In other words, it is strongest where texture in the image changes very rapidly over a short distance, such as the fabric in the man's tweed coat or the sharp edges of his profile. Exactly which type of texture becomes emphasized is controlled by the USM radius parameter. A larger radius would mean that texture which changes over a larger distance (in pixels) will be emphasized more.

In the old man example, one also can see exactly which areas are most affected by the unsharp mask (lightest regions) by looking at the b/w mask itself:

So the entire image *is* effected-- it's just that some regions are impacted much more than others due to the mask effect. If we made the radius in the old man example much larger (say 10 pixels in the size shown on the web), then you would start to notice that the unsharp mask would influence the background texture a whole lot more (and the fabric in the man's shirt a whole lot less).