How to Spruce Up a Scanned Halftone in Photoshop CS6

Adobe Photoshop Creative Suite 6 has a way to improve the quality of a scanned image when you do not have access to the original.

Publications 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 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 onscreen. To get at least some of that subtlety, photographs have to be converted to a pattern of dots before they can 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 sometimes actually blurs your image more than you want.

Fortunately, you can usually do a pretty good job in Photoshop. But remember that, if there’s any way possible, scanning from continuous-tone images always yields the best quality.

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 while you work.

    image0.jpg
  3. Choose Filter→Blur→Gaussian Blur.

    The Gaussian Blur dialog box appears.

    You can also first convert your image to a Smart Object and then apply your Gaussian Blur and Smart Sharpen filters as Smart Filters.

  4. Move the Radius slider to the right until the halftone pattern is blurred, and then click OK to apply the blur.

    image1.jpg
  5. Choose Filter→Sharpen→Smart Sharpen to restore some of the image’s sharpness now that you’ve eliminated the pattern. Choose Gaussian Blur for the Remove algorithm.

    As long as the Radius value for the Smart Sharpen 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.

    Try to find a setting that does the job without making the image appear unnaturally sharp or show too much contrast.

    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.

    image2.jpg
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