retinax
Well-known
Like an unsharp mask!But it does it by raising the contrast of adjacent tones.
I'll concede that it does produce something that resembles a good silver print in that contrast in the mid-tones can be high while keeping highlights and shadows in check. But the means that this can be achieved with in the darkroom, dodging and burning, or good lighting before that, are also available in a digital workflow and using them manually rather than letting a algorithm apply something all over the picture usually gives better results.
To me, pictures with significant "clarity" added (as well as darkroom prints with excessive unsharp masking) look quite unpleasant. HDR has the same issues. Hard to put into words, but some of the things it does for my perception: Destroy the "rhythm" by giving contrasty detail everywhere rather than just in specific areas, putting too much detail in shadow or highlight areas that should be low contrast, halos, unnatural look to specific things like faces, even toned areas get gradients, depth perception from haze in the distance is taken away... these things become painfully noticeable once your vision has learned to detect them. Of course tiny amounts of it can be acceptable, that's like sharpening or the differences between a contrastier and less contrasty lens.