If you are interested in sharpening, take a look at this lesson by, probably, the world's greatest authority on the matter.
Well, that depends on who is calling whom an authority (for a blind person, a Cyclops seems to be a genius).
His conclusion, briefly, is that most software has got it totally wrong (or worse).
Actually, he is right, but for the wrong reasons. He does have a point about the almost(!) brain-dead controls we have to cope with in many applications.
Unfortunately in this time and age, not a single time did he mention deconvolution
(which might be a good thing if he doesn't understand what that is about). It's a basic procedure to anybody even remotely familiar with the physical properties of digitized
image data (from a CCD/CMOS sensor, or even from an analog scanning tube). I do understand where he's coming from, with a scanning operator's background, and from that limited angle of view, he is correct, modern controls (wang-bars) do suck.
However, anybody who is even slightly introduced to digital signal processing (DSP), and could be
for free, should know that digitized image data offers a different toolset to reduce Capture deficiencies (residual lens aberrations and diffraction, and DOF blur), which will impact the entire(!) spatial frequency range of the Modulation Transfer Function (MTF). In addition to that (capture sharpening in a gamma adjusted, or not, colorspace), there are several methods to address output sharpening, and the better ones do address variable sharpening at different tonality or local contrast ranges.
For quite some time already, I've been an advocate for luminosity blend-if layers, to control potential clipping artifacts from poorly designed sharpening tools (see the dialog box below which only suggests a starting point for adjustment):
Proper deconvolution sharpening shouldn't even produce clipping (or noise amplification, if avoidable), but the current sharpening controls usually do not offer any control (or even really useful user guidance/feedback) to that (clipping) effect.