Selectively Applying a Filter in Photoshop

You don’t need to apply filters to an entire image or entire layer. You can achieve some of the best effects when you apply a filter to only a portion of an image, say to an object in the foreground, but not on the background. Your choices include
- Selections: Make a selection and apply the filter only to that selection. You can use Quick Mask mode to paint a selection. This technique can give you a high degree of control; it even lets you feather the edges of the selection so that the filter effect fades out.
- Channels: You can store selections as alpha channels, visible in the Channels palette, of course. But you can also choose to apply a filter only to one of the other channels, such as the Red, Green, or Blue channels in an RGB image. This is a good way to create a filter effect that’s applied to only one color in an image.

Sprucing Up a Scanned Halftone
Publications such as newspapers and magazines use only a limited number of ink colors to reproduce a photograph. Every tone you see in a black-and-white image must be reproduced by using pure black ink and the white (okay, dirty beige) of the paper. Full-color images are represented by combining CMYK (cyan, magenta, yellow, and black). Printers (both the commercial kind and the kind that sits on your desktop) can’t use various shades of gray ink to create grayscale photos. They also can’t use different strengths of color inks to generate the rainbow of hues you see in an image on-screen. Instead, photographs have to be converted to a series of dots in order to be printed. Our eyes blend the dots together to produce the illusion of a grayscale or color image with smooth gradations of tone.

However, a problem arises if you want to reuse a photograph and don’t have access to the original. Scanners can capture the halftone dots, but the resulting image usually has an unpleasant pattern, called moiré. You have several ways of reducing the moiré effect, usually by blurring the image so that the dots merge and the underlying pattern vanishes. Many scanners have a descreen setting that partially eliminates the effect, but that setting may actually blur your image more than you’d like. Fortunately, you can usually do better in Photoshop. In the following steps, I demonstrate an easier way. Over the years I managed to lose the original negative and print. You can see the dreadful moiré pattern that results when I scanned the clip. To rid your scanned halftone print of bothersome moiré, follow these steps:
1. Open the image in Photoshop.
2. Zoom in so that you can see the halftone pattern clearly as you work.
3. Choose Filter-->Blur-->Gaussian Blur. The Gaussian Blur dialog box appears.
4. Move the Radius slider to the right until the halftone pattern is blurred, and then click OK to apply the blur.
5. Choose Filter-->Sharpen-->Unsharp Mask to restore some of the image’s sharpness now that you’ve eliminated the pattern.
As long as the Radius value for the Unsharp Mask filter doesn’t exceed the radius of the Gaussian Blur that you first applied, the two filters won’t cancel each other out. You want the sharpening to make the details of the image crisper without bringing back those blurred halftone dots.
6. Move the Amount slider to the right to sharpen the image.
When working with your own photos, you may need to use different Radius and Threshold values. In most cases, leaving the Threshold at 0 is fine. Calculate a starting Radius value by dividing the ppi (pixels per inch) of the scanned image by 150. If a moiré pattern reappears, reduce the Radius value.
7. Click OK to apply the sharpness.