An Alternative Method for Sharpening Images

One alternative to using the filters in the Sharpen submenu is to use the High Pass filter. The filter works by increasing contrast on the edges of the image and leaves large, flat areas unaffected. The filter has only one setting: Radius. To apply the filter, take the following steps:

1. Duplicate the layer that you want to sharpen and give it a descriptive name, such as High Pass.

2. Set the magnification to 100% for screen output or to 50-25% if outputting to an offset printer.

3. Choose Filter^ Other^ High Pass.

4. Apply a low Radius setting if the image contains a large number of edges, such as a scene containing foliage, or a larger value if the image contains fewer edges, such as a portrait. Your duplicate layer should turn a mushy gray, but you should be able to discern the edges through it (Figure 11-25). Decrease the Radius if grain or noise starts to materialize in broader areas of color.

5. Click the OK button to apply the setting and to exit the dialog box.

6. Back in the Layers palette, change the blend mode of the High Pass layer from Normal to Overlay. Soft Light and Hard Light can also be used; each produces a slightly different result.

To refine the filter effect, decrease the opacity of the filtered layer, or use the Blend If sliders to localize the effect (Layer ^ Layer Style ^ Blending Options).

Figure 11-25: Using the High Pass filter to sharpen a noisy image and the result.
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