If you've just come in, start with my first post on Sharpening in Older Posts.
I said that image sharpening and image resolution are different. Resolution is concerned with the discernment of image detail. So what is sharpening about then?
Juxtaposing black and white to make an edge more conspicuous is a trick nature discovered hundreds of millions of years ago. It evolved both in plants and animals for a number of purposes.
Flowers may use it to help attract and guide pollinating insects to nectar and pollen. Note how stark black and white zones each side of an edge makes the edge stand out.
Foxglove nectar guides
Dog violet nectar guides
Animals often use it to attract a mate or to send signals to each other. The clearer the advertisement the more effective it will be, so exaggerating edges in patterning increases the likelihood of being noticed and attracting the required response.
Animals can't blur themselves, but one way of reducing edge contrast is by being patterned like the background. Camouflage helps prey animals avoid being noticed. Some animals like chameleons and octopus' can change their appearance very quickly to fit in with their surroundings.
A very common ploy for reducing edge contrast is obliterative counter-shading.
Because animals are normally lit from above, they look lighter on top and darker in the shadow underneath. Many animals counteract this by having lighter coloured fur, feathers or skin on their tummies than on their backs. From a distance this flattens their appearance and helps conceal their edges.
These ibexes are almost invisible.
The Luna Moth normally hangs upside down and is difficult to spot (top picture). Turning it the other way up reverses the counter-shadowing making it very conspicuous.
Reverse counter-shading can be found in some animals. This makes them easier to see and is a warning to others to steer clear because it has a nasty sting, bite or smells obnoxious. Skunks and Honey Badgers have reverse counter-shading. They are lighter on top.
This is a Honey Badger - they stink and are very aggressive.
Finally a couple of examples from art and graphic design.
The Black Bow, a Conté drawing by George Seurat.
The use of a drop shadow helps make text stand out.
My take-home message is that manipulating edge contrast is not some modern innovation dreamed up by Adobe programmers - nature has been making widespread use of it for a long, long, long time.
The final blog on sharpening will explain (at last!) how to adjust edge contrast in your pictures to make them look sharper, including what each of the slider controls in Unsharp Mask do.
While the imperative of art is toward a revealed truth, the execution of art is a concealment of that truth through beauty. Oh, sorry, money worries this week. Yeah. Sorry.
Psychic Bob in The Daily Mash