I don't know about you, but I think the Luminance Smoothing control wins the Photoshop "Cool Name" award for adjustments. Luminance is an interesting word to begin with, but when you combine it with smoothing, you sound impressively technical and artistic to your friends. What it does is even better. Luminance smoothing reduces one of the two types of noise that can appear in an image: noise contained in the grayscale areas.
All digital camera sensors produce some sort of noise. It's not noise like someone yelling at you to take out the garbage, but electrical noise: graininess caused by an electrical field affecting the image sensor while it's collecting light. In most photos, you wouldn't even notice the noise, but for those taken at high ISO settings (400 to 1600) or long exposures, you can see the graininess when you zoom in on your image at 100 or 200 percent (as in the area shown in Figure 9-22). Visible noise. What a concept! And you won't even see noise in your viewfinder; it sneaks up on you later, like the boogie man.
The Luminance Smoothing control reduces that grainy-looking noise lurking in the shadow parts of an image. Here's how to use it for that purpose:
1. Zoom in 100 to 200 percent by using the Zoom tool or by pressing Ctrl+ (&+ on a Mac).
2. Use the Hand tool to move the Image Preview around until you can see a good sample of luminance noise that would usually be found in the dark areas of the image.
3. Move the Luminance Smoothing slider to the right to reduce luminance noise (as in Figure 9-23).
For this image, I moved the Luminance Smoothing slider rightward to a setting of 30, which reduced image noise to an acceptable level.
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