There's one more reason we sharpen, which is to tell the story we want to tell, and that often involves improving reality. The concept of improving reality is anathema to some photographers, and if you're one of them, feel free to skip this section, but do recognize that it's what some other photographers get paid to do!
We sometimes want to call extra attention to an element in an image by making it appear a little sharper than its surroundings. Head shots often benefit from a little extra sharpening around the eyes, for example. I call this kind of sharpening "creative sharpening" because unlike the other kinds of sharpening I've discussed so farsource-sensitive, content-sensitive, and output-sensitive sharpeningcreative sharpening can't be automated. It requires manual application and human decision making.
Figure 2-26 shows two images before creative sharpening on the left, and after creative sharpening on the right. On the top image, I added a little extra sharpening to the eyes and hair. On the bottom image, I added extra sharpening to the wall to reveal the cracks and texture, but not to the doors or the ground.
Figure 2-26. Creative Sharpening
There are really no hard and fast rules, beyond those imposed by good taste, regarding creative sharpening. Obviously, if you overdo it, you'll wind up with a crunchy image, and if you don't do it enough you'll waste time doing things that don't show up in the final image, but within those bounds you have a lot of leeway.
A reasonable rule of thumb is to apply creative sharpening in such a way that it doesn't stick out from the rest of the image as an area that has obviously been more heavily sharpenedthe transition between the areas that have received creative sharpening and those that have not should be imperceptible at Actual Pixels (100%) zoom or lower. The smoothness of the transition is more important than the actual appearance of the pixels on the screen unless you're actually sharpening for on-screen viewing.