What *exactly* is Clarity?

[ThomasT]

Hi,

camera raw and lightroom have the slider clarity.

But what exactly is clarity?
Yes I know what it is in principle, its mide ton contrast, but I'm asking for the algorithm to reproduce it in PS.
In the internet you read a lot about its like unsharp mask with large radius and small intensity. But this is true only for small corrections.
When moving the clarity-slider to the extrem right the image looks different. At first it seems that it influences only the luminosity.

But using an upper layer, applying unsharp mask (radius around 100, amount 20% -30%) and combining it with luminance-method looks quite differently.

I tried differenz things. Some come near, but not exaclty. And all what I found in the internet is rubbish and ad hoc thinking.

I need the hard slightly desaturated look when pushing clarity to the extreme.

[jbm007]

True

But is depends on the subject matter.

Its biggest issue is that it is like a blunt knife.

Hard to control what it does if its not limited in its use.

[Paper_Mine]

I prefer Dehaze than Clarity to avoid photos look like over-sharpened.

[Camera Raw Fan]

YES! 

[Trevor.Dennis]

I've always thought that the histogram tells us a lot about how Clarity affects an image. Especially the Expanded Histogram showing separate channels. It depends on the source image of course, but it is safe to say that it flattens the curve and boosts shaddows and highlights.  Similar to moving the blacks slider to the right blocking out shadow areas, and boosting highlights, but tries to contain things within blowing out highlights (especially) and blacks, which I think it more inclined to allow loss of shadow detail.  You can see in the example below, that the shadow end is off the chart  — so to speak.

Before (No Clarity)

After (Clarity 100%)