I've never seen this explained -- despite years of my sharpening images! These directions are for a generic unsharp mask too, Aperture has additional options.
Photojojo » Unsharp Mask: How Do You Actually Use That Thing?I'm going to add this site to my feed list.
... Step 1: View the image at 100%. Set the radius between 1 and 3. Set the amount between 300 and 500. Set the threshold at 0.
This will look like crap. But you’re going to fix it in a minute, so don’t worry.
Slide the radius level up until you start to see nasty halos forming, then back it off a bit. It’s OK if it looks a little bit harsh at this point.
Step 2: Change the image view to 50%. Adjust the amount until it looks grainy and oversharpened, than back it down a little.
Since web images need a fairly high amount of sharpening (in the 300 to 500 range), our example here isn’t quite as dramatic as we’d like. We made the “after” image a little soft so you can see what’s going on at this stage.
Step 3: Move the threshold slider up until the low-contrast areas look smooth, but you can still see fine details...
... Photography Jam has a good set of starting points for different kinds of pictures. We liked their all-purpose and web settings, but there are lots more on their site.
All-purpose sharpening: amount=85, radius=1, threshold=4
Sharpening for the web: amount=400, radius=0.3, threshold=0...