I have answered a question posted by Glenn Kroeger in the past. Because of my interest on the matter, I post again to know some opinions about USM. Thanks.
Some points about USM,
a) Any digital image, both from scanners and digital cameras, will need some amount of USM in order to be sharply viewed at final step (screen, inkjet, photogrphic paper, etc.). There are a lot of particular characteristics of digital imaging process that induces lost of sharpness.
b) The amount of USM necessary for a given picture depends on, essentially, three questions: The first is the scanner or digital camera own performance. The second arises from the amount of present detail (image frequency) in a given picture. The third, finally, is related with the final step of visualization (screen, inkjet, photogrphic paper, etc.).
c) All the stated above concludes to the impossibility of the existence of any rule of thumb about wich is "the best" amount of USM or "the best" smart software to apply it.
d) There are at least two questions to have into account: It seems obvious that we need some test of our equipement (scanner and/or camera) to try the best amount of USM to apply to our pictures. In second place we need to analyze EACH picture in particular and the amount of texture and detail to decide the appropriate amount of USM to be applied. It is too important to realize if it is necessary or advisable to apply the USM or the same USM to the whole picture area.
e) Using the USM tool a gaussian blur is applied to a copy of the original image to produce a new image with no fine detail; the blurred image is then substracted (mathematicaly) from the original image to extract the fine detail. This fine detail is added to the original to highlight the detailed areas.
f) The RADIUS determines the amount of gaussian blur first applied (Probably large files need more radius than small files to be the same effect). The AMOUNT defines de quantity of detailed version added to the original. The THRESHOLD indicates how different in brightness (from 0 to 255) must be two adjacent pixels to enables the USM be applied.
g) The threshold permits to avoid some areas to be contrast enhanced by the USM, like the face skin in a portrait or the sky in a landscape. May be interesting to analyze previously the neighbor pixel differences in those areas with the INFO Palette in order to decide the thershold to be applied.
Disagreement or discussion will be welcome,