Another way to sharpen a lot ...

Many editors allow one to use Unsharp Mask (USM) selecting pixel radius and amount at least ... but only once, for example RawDigger. Doing it again in those editors replaces the old USM adjustment with the latest one.

Other editors place new adjustments on top of older ones; so it is that, in those editors, USM can be applied several times which brings an advantage or two. For example, if one has captured an overly soft image, it can be rescued by selecting a largish radius and a lowish amount; which increases local contrast rather than actually sharpening edges. Then go USM again at a smaller radius but larger amount ... and again, say one or two more times reducing the radius and increasing the amount each time.

Here's an image with USM applied to a selection in the middle four times: 5px, 3 px, 1.5 px, 0.7 px. Amounts somewhat exaggerated for the purpose of illustration:

Comments welcome ...

Halos were caused by exaggerating the amounts.

Same selection, lower radii and amounts rescued this overly soft image:

Of course, halos can be avoided by being much more careful with choice of radii and amounts than I was in this thread. In other words, the method itself does not always cause halos.