The creative possibilities for detail enhancement in digital imaging are simply staggering— and growing more so every day. Before their advent, few dreamed of having such possibilities, while many didn'tdare. Today, contemporary practitioners perform yesterday's miracles daily. Given the newness of the technology, it's not surprising that some find themselves arriving at a destination far astray from their intended target, whether consciously or unconsciously. Some don't go far enough, while others go too far. Jeff Schewe, author of the definitive resource on the subject Real World Image Sharpening, is fond of saying, "Under-sharpening is a venial sin while over-sharpening is a cardinal sin." There is a cure, and it doesn't involve penance; it's much simpler, less painful and more effective than that. Learn what to look for and take appropriate measures.

Whether you're testing or applying sharpening, view your images at 100% screen magnification to accurately assess detail. Zooming in or out to any other screen magnification may hide artifacts and, in some cases, display banding onscreen where none exists. 

So what does sharpening an image do anyway? Simply put, it accentuates line and texture through contrast. That's all any of the sharpening filters do; they just produce different effects (desirable) and different artifacts (undesirable).

When assessing the strengths and weaknesses of detail-enhancing tools, it's useful to look closely at the two basic building blocks of detail: line and texture. Whether thick or thin, a line can be light or dark; often the two exist together. Texture can be divided into different frequencies of detail: high (fi ne), medium (coarse) and low (smooth).
  Do all images always benefit from noise reduction and sharpening? Most do. Still, I recommend, "Always avoid saying always and never say never—except when saying that."

Similarly, most images benefit from noise reduction—or sophisticated blurring. Reduce noise before sharpening. If you reduce noise after sharpening, it's likely that you won't go far enough while sharpening and you'll undo some of sharpening's benefits.

Some would like to use one setting to sharpen all images. This commonly produces suboptimal and sometimes disastrous effects. To achieve optimal results, you simply can't sharpen all images equally—because not all images are created equally.

Different images may be more or less well focused and have more or less depth of field.

Different images may be exposed at different ISOs, so separating noise from micro-texture may be more or less difficult.

Different images may contain different frequencies of detail; they'll benefit by being sharpened selectively, sometimes with different settings and even different filters.

Different people may like their images softer and smoother or sharper and more textured; what balance is finally struck depends on the individual, the image and what's being said.

While automation after testing can be used effectively at certain stages in a digital imaging workflow (sometimes during capture sharpening, rarely during creative sharpening, often during output sharpening), there's simply no substitute for looking closely and responding sensitively, especially if you want your images to convey your individual ways of seeing. 
  Identifying and developing a sensitivity for the artifacts digital sharpening produces will help you choose a sharpening method and what settings to use during any stage of your sharpening workflow. You can easily see the artifacts digital sharpening produces by overdoing it. Apply a filter like Unsharp Mask at maximum strength and look closely at what happens.

Following are the seven most common digital sharpening artifacts. These artifacts can be reduced in one or more ways. Here's a list of options for each:

1. Noise
Raise Unsharp Mask's Threshold.
Use High Pass sharpening.
Blur High Pass layers.
Mask select image areas.

2. Exaggerated Texture
Reduce Unsharp Mask's Amount.
Use High Pass sharpening.
Blur High Pass layers.
Mask select image areas. 

3. Visible Light Halos
Reduce Unsharp Mask's Radius to make halos thinner.
Reduce Unsharp Mask's Amount to make halos darker.
Set the Blend Mode of the Unsharp Mask filter or layer to which it's applied to Darken.
Use High Pass sharpening for softer, more feathered contour accentuation.

4. Visible Dark Lines
Reduce Unsharp Mask's Radius to make halos thinner.
Reduce Unsharp Mask's Amount to make halos darker.
Set the Blend Mode of the Unsharp Mask filter or layer to which it's applied to Lighten.
Use High Pass sharpening for softer, more feathered contour accentuation. 
  5. Loss of Highlight Detail
Use a sharpened layer's Layer Styles/Blend If sliders to recover it.
Mask the highlights.
  6. Loss of Shadow Detail
Use the Blend If sliders in Layer Styles to recover it.
Mask the shadows.

7. Increased Saturation
Change the blend mode of the fiter or sharpened layer to Luminosity.
Desaturate High Pass layers.
 

Conclusion

If you know what to look for, you'll know what path to choose and how far down it to go. Training your eye for what to look for and understanding the upper limits of what other people find to be naturalistic, or at least not distracting, is the first step to developing your unique sharpening style. The second step is learning how to produce certain effects and avoid others with the tools at your disposal. Once you've taken these steps, you can take the third and final step, knowledgeably putting craft in the service of your vision to make compelling visual statements. Enhancing detail is one area of expertise that's well worth mastering for all photographers.