Absolute Beginner's Guide to Digital Photography

For portraits or images with people, make sure you zoom in on the individual’s face

and experiment with threshold. Sharpening will create a texture in the skin, which

should be subdued or avoided if possible.

Taking Advantage of Blur

More often than not, images are blurry. Rarely are they too sharp. So why are the

blur filters used just as often? The reason is blur filters are the only way to remove

moiré patterns and halftone dots that occur when you scan printed material. You

also might want to blur selections to make them blend with the image.

Removing Moiré Patterns and Halftone Dots

Suppose you need to scan an image from an old brochure or magazine. If you scan

the image, there are three possible results: the actual halftone screen used by the

printer will appear, a moiré pattern will appear, or both will appear in the scanned

file.

A moiré pattern is an interference pattern produced by overlaying similar but

slightly offset templates (see Figure 13.7). The printer uses a screen—a metal plate

with a pattern of holes—to produce the printed material. Your scanner has its own

pattern that it uses to scan. The two screens usually interfere with each other, creat-

ing a moiré pattern.

CHAPTER 13 THE IMPORTANCE OF FILTERS 197

FIGURE 13.7
Examples of
moiré patterns.

The easiest way to reduce or remove a halftone screen pattern or moiré pattern

when scanning is to turn off auto-sharpening. Most scanner software will automati-

cally sharpen a scan to compensate for the imperfections of the scanner hardware.

Turn off this feature if possible to reduce these patterns.

To reduce or remove the effects of moiré and halftone screens in Photoshop, use the

Gaussian Blur filter. Gaussian Blur smoothes differences in an image by averaging

pixels next to the hard edges of defined lines and shaded areas.