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If Images Look Oversharpened - or
Sharpening Images with Dark Repetitive Detail in Photoshop
category: digital photography and image editing
Photoshop's Unsharp Mask (USM) filter creates the illusion of focus in an image by finding the contrasting edges and adding a dark halo on one side of each edge and a light halo on the other side. The width of these halos in controlled by the Radius slider, and the brightness of the halos by the Amount slider. Threshold adjusts the level of contrast required between pixels before an area is treated as an edge, and is intended to reduce sharpening grain or noise. Threshold isn't perfect and can create rough transitions between sharpened and unsharpened areas. This problem is discussed in (link) Alternative to an Edge Mask and in (link) Sharpening with an Edge Mask.
The halos themselves, especially in images with very dark or very light repetitive detail, or those with bright colors against a light background, can be distorting.
One problem is color distortion in the light side of halos introduced by USM, especially along the edges of bright colors. One way of avoiding color distortion requires going to Lab Color, so you will have to save your master and flatten the image to be sharpened or you will lose adjustment layers in this otherwise only slightly destructive transfer. When your image is in Lab Color, switch to the channels palette and click on the Lightness channel. Sharpen the Lightness channel with USM and return to RGB mode. No sharpened colors results in no bleeding of red into pink at the edge and so forth.
Another way to deal with color distortion, without transferring to Lab Color mode and losing adjustment layers, is to link(Stamp) visible layers into a new top layer, and run the USM filter, choose Edit, and then Fade Unsharp Mask. Set the Blend Mode to Luminosity in the Fade dialog dropdown.
After setting the Blend Mode to Luminosity in the Fade USM dialog dropdown, if you find the light side of the sharpening halos are distorting in themselves, as might be the case with waves in dark blue water or bricks in a wall, set the Blend Mode of the stamped and sharpened layer to Darken, or in the reverse case of lighter detail being distorted by the dark halos, set the Blend Mode to Lighten.
I like to go all the way and stamp a layer, right on top, in my image master file, duplicate the stamped layer, and sharpen that layer with USM. I then set the Blend Mode to Luminosity in the Fade USM dialog dropdown while it's still available, just after sharpening. Then I duplicate the sharpened and Faded layer, set the original to Darken and copy to Lighten. I can then adjust the opacity sliders of each sharpened layer until I see what I want.
If your image is noisy or grainy, combine this last with an edge mask that, when inverted, acts as a perfect mask for using the Gaussian Blur or Median filters to clean up the noise.
He said on 06.15.05 @ 04:23 PM CST
