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The term "unsharp masking" (USM) originated back in the days when color separations where done on process cameras and and later on drum scanners. Been there, done that, in the days before Photoshop.
What it does is change the contrast at tonal boundaries. These are 800% screen shots before and after USM was applied...
RustyBug has explained the basic controls. By way of analogy, think of USM creating a stone wall at the tonal boundary. Amount will make wall higher, Radius will make it wider encroaching on more real estate on either side.
The method I use was learned about 10 years ago from a Dan Margulis article in a magazine. It involves two steps: first applying USM, then using the Edit Fade function to adjust it with an interesting twist. The fade step is applied in Luminosity mode which thanks to some deep Photoshop mojo applies it to the L channel at the color management level.
As a first step I applied 500, .2, 0 sharpening overall. That is the maximum amount, but applied very narrowly to the tonal borders (i.e. a high, narrow wall). Then I used Edit > Fade to scale it back, changing the mode in Fade from "normal" to "luminosity". Due to the way Photoshop works deep in it's bowels changing the mode to luminosity applies the sharpening only to the L channel as if in Lab mode even when the file mode is RGB. It eliminates any color artifacts that might occur when two contrasting primary or secondary colors are sharpened in RGB.
The more useful think the Fade Step does is allow you to adjust the amount if USM interactively while seeing the full image in the edit window by moving the Opacity slider from 0% (no sharpening applied) to 100% (what is seen before the fade step).
The amount of USM needed varies depending on the size of the image and how much detail it has. I use the high amount / low radius method for my small screen images because I like how it restores the crispness without any halos. But I hardly ever apply it at 100%. I move the opacity slider back and forth and usually wind up around 60-70%. The edit above is at 85% which is a bit more than I'd normally use for a portrait. Also I wouldn't apply USM overall for a portrait.
For portraits I have an action I use which creates surface blur, high pass sharpen, and USM layers with masks which I selectively open and then blend the layers to adjust the appearance of the skin and tone down distractions like the buttons by blurring them a bit to take off the "sharp edges". Here's the high res copy with that treatment applied..
BTW - the skin highlights are blown out in the the red channel of that image due to either overexposure at capture or from the 16 bit wide gamut > 8 bit sRGB conversion step. You need to allow "headroom" at capture for the editing workflow changes. In addition to applying the skin treatment I fixed that and cloned out the less than ideally placed nose shadow that was hanging out because your key light was a bit to low...
I apply USM for printing differently than for small screen images. Something to realize about USM for prints is that you really can't judge it on screen because it needs to compensate for variables of printing. The best way to zero in on a good USM baseline for prints is to take an image, apply various amounts and methods of USM, make prints, put them on the wall and then look at them from different distances.
Viewing distance is a variable affecting perception of sharpness. Beyond reading distance of 18" the brain interprets 3D shape based on overall contrast differences between planes, similar to a wireframe rendering. As a general rule as you make a print larger and increase the viewing distance you can increase the amount of sharpening.