printing - How should one sharpen differently depending on the target usage for a photo?

In an answer to another question, a statement is made:

Sharpening is the only (? I can't think of any others ?) edit which needs to be done differently depending on the target medium. E.g. you would sharpen a 20"x10" print differently from a 640x480 pixel image for a web page.

What should be taken into account when sharpening for various output methods? How would one sharpen for a print vs. an onscreen image vs. other uses and how does the final image size affect sharpening?

Edit/added: with printing, does the surface/material of the print (paper, metal, canvas, etc) affect how much one should sharpen?

Answer

It's mostly the resolution of the image that affects the sharpening. When using unsharp mask, a radius of 0.1 mm is a good starting value. For an image displayed on a computer monitor at about 100 PPI, that translates to a radius of about 0.5 pixels.

Then there are also differences in the media that causes the sharpening to have varying effect. A printing processes may for example have a slightly blurring effect, so you would have to over-sharpen the image slightly to compensate for that. Some display screens are a bit blurrier than others (e.g. a CRT screen is blurrier than an LCD screen).