The Moiré Pattern

Halftones are incredibly useful, but it does have some side effects. Since every printed material like a newspaper or magazine is created with this process, although the naked eye can't see it, scanners are affected.

Anytime an image is scanned into the computer, there is a crosshatched or dotted pattern all across the image that is produced on the screen.

This is called a moiré pattern. In a scanned image, these patterns are caused by interference between the two sets of fine pattern grids the scanner samples and the halftone screen in the original image. Any scanner will do this, it's a simple fact of life. There are ways around this visual pattern, but it is something anyone should be prepared for.

(image from Australian Digital Camera magazine by Barrie Smith in 1998)