Cranking Up the Focus with the Sharpen Tool in Photoshop

In theory, the Sharpen tool is nothing more than the Blur tool in reverse — instead of decreasing contrast among pixels, the Sharpen tool increases the contrast. In practice, however, you need to use this tool with a bit more care. Where blurred areas tend to fade from a viewer’s notice (at least in terms of how our eyes perceive them), sharpened areas of an image jump out at people. If you blur an area a little too much, you may not even notice. But even a small area that has been oversharpened can change the entire appearance of an image — and not flatteringly.

Sharpening an Image (With Care)
You can often successfully sharpen small areas with the Sharpen tool. Sometimes the eyes in a portrait can benefit from a little sharpening. Or, you might want to sharpen an area to make it stand out more distinctly against a slightly blurred background. Here are the simple steps to follow to use the Sharpen tool:
1. Choose the Sharpen tool from the Tools palette.
Press the R key to select it directly if it’s the last focus tool used. If not, press Shift+R to cycle through the Blur and Smudge tools to activate the Sharpen tool.
2. Select the brush of your choice from the Brushes palette.
3. Choose a blending mode from the Mode pop-up menu.
4. Choose the strength of the sharpening effect with the Strength slider or text box.
Using a fairly low value (say, 25 percent or less) is a good idea because you can build up sharpness slowly, being careful not to overdo it. You know you have gone too far with the sharpness when the pixels start to look noisy and grainy.
5. Use the information on all your layers for Photoshop’s contrast-increasing algorithms by selecting the Use All Layers option.
6. Paint over the areas you want to sharpen.
7. When you finish, choose File-->Save to store your image.
Sharpening increases contrast, so be careful when using the Sharpen tool if you plan to also adjust the Levels or Curves controls. Any change that increases contrast in the whole image also boosts the contrast of an area you’ve sharpened. The Unsharp Mask filter offers more options and better overall control, so unless you really need to paint the sharpening effect, you’re usually better off using the filter. If you really want to apply the effect with brushstrokes, you can always apply the Unsharp Mask filter to a whole layer, take a snapshot, undo the filter operation, and then use the snapshot as a source to paint from using the History palette.