Sharpening Images in Adobe Photoshop CS4
- Thirteen. Sharpening Images
- #93. Using the Smart Sharpen Filter
- #94. Using High Pass Sharpening
- #95. Sharpening Selectively
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There is something incredibly compelling about photographs that are vivid and sharp. It’s as if the sharpness draws us in, making us want to look closer. As you strive to create sharp images, you stand on the shoulders of many great photographers, including Ansel Adams, who pioneered and evangelized the idea that precision and sharp focus are fundamental to good photography.
While Ansel’s pursuit to capture sharp images was focused on the camera and printing, you now have an even bigger advantage—Photoshop, which provides a number of sharpening filters. This chapter focuses on the most useful of these filters.
Keep in mind that no one sharpening filter is best, and no one sharpening technique works for every image. It’s important that you be familiar with and practice using all of them so that you have a broad base of sharpening skills that you can apply, as appropriate, when fine-tuning your images.
It’s also important to remember that it’s best to incorporate sharpening as the last step in your imaging workflow. Completing all other enhancements, such as retouching, color and tone adjustments, image enhancements, and resizing before you sharpen is critical. This approach will deliver the best sharpening results for that particular image size and resolution.
Of course, Ansel was interested in more than just the fundamentals of precision and sharpness. His goal was to create strong and compelling photographs. As he once said, “There’s nothing worse than a sharp photograph of a fuzzy concept.”
As you learn how to sharpen images in Photoshop, remember not to lose sight of this ultimate goal: to create images that have impact. If you do lose sight and end up oversharpening your images (a predicament common to those who are new to digital imaging), you run the risk of calling attention to the sharpness rather than the actual images.
#92 Using the Unsharp Mask Filter
One of the most useful sharpening tools Photoshop provides is the Unsharp Mask filter. (Yes, the name sounds like it does the opposite of sharpening—it’s derived from an old darkroom technique where the name does make sense.) This filter scans the image for areas where pixels whose tonal values differ by a certain quantity (the “threshold,” chosen by you) are right next to each other. These areas tend to be edges, and the filter lets you sharpen these edges by increasing the contrast of pixels within those areas. In other words, you lighten the light pixels and darken the dark pixels, and the boundaries between them become more pronounced.
Sharpening an image involves a bit of “give and take.” While you want to increase the overall sharpness of the image, you do not want to increase or exaggerate noise or artifacts. Fortunately, the Unsharp Mask filter has controls that help you specify an ideal amount of sharpening without degrading the overall image quality. Use this filter, as opposed to the others, when you’re interested in making precise sharpening improvements to your image.
To use the Unsharp Mask filter:
- Select the layer you want to sharpen.
- Choose Filter > Sharpen > Unsharp Mask. The Unsharp Mask dialog box appears (Figure 92a).
- Select the Preview option and adjust the Amount using the slider or enter a percentage.
The Amount setting controls the overall intensity of the sharpening. The value you specify determines how much to increase the contrast of the pixels. The actual Amount depends on your image’s size and resolution. For medium- to high-resolution images, try an Amount of 100%. For higher-resolution images, try a higher percentage.
- Adjust the Radius setting, which controls the “reach” of the sharpening. If you are sharpening any edge, for instance, the
Radius setting defines how far the sharpening will extend on either side of this line. In more specific terms, the setting
determines the number of pixels surrounding the edge pixels that will be affected.
The greater the Radius value, the wider the edge effects will be, and the more obvious the sharpening. So your Radius amount will typically be low. For example, for images destined for Web pages, try a Radius between .1 and .3. For high-resolution print images, try a radius between 1 and 3.
- Adjust the Threshold setting. This setting “saves the day” by limiting the extent of your sharpening. When the filter evaluates
a pixel for sharpening, it compares its tonal value with that of its neighbors. If the difference is less than the Threshold
setting, nothing happens. If the difference is greater, the contrast between the pixel and the ones around it is increased.
When the Threshold value is set to 0 (the default setting), if there’s any difference at all between tonal values, sharpening is applied. This means that even tiny fluctuations in tone, such as image noise and film grain, will be exaggerated. Raising the value above 0 means that only more defined features, such as edges with contrast, will be sharpened, whereas broad areas of near-uniform texture, like skin and sky, will be protected from sharpening.
The appropriate setting will vary, depending on the nature of the image. The more detail there is, the lower the setting. For starters, try a value between 2 and 20. Images with a lot of detail usually require a low setting; use higher settings for images with less detail (like portraits).
- When your sharpening appears to look its best, click OK to apply the sharpening.
Figure 92b shows the before and after versions of a sharpening project.
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The Halo Effect
Oversharpening can produce a halo effect around the edges of a subject that has been sharpened (Figure 92c). Look closely at your image, and if you see the halo effect, reduce sharpening.
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