Descreening

Why descreen?

If you've ever scanned an image from a book, magazine or newspaper (printed images) then you may have noticed that the scanned image consisted of several small circles that were visible and quite distracting in the image. That effect is called Moiré. It is a pattern which emerges from the image which is part of the mechanical technique for printing images. It's existence in graphic arts has been clarified by the fine contributors to Wikipedia as the result of "the usual technology for printing full-color images [which] involves the superimposition of halftone screens. These are regular rectangular dot patterns—often four of them, printed in cyan, yellow, magenta, and black."

Below are examples of an image scanned from a newspaper, one having been descreened, the other was not. Click to enlarge the image

So, when should I use the descreening adjustment?

You shouldn't always use the descreening filter when scanning images. The best rule of thumb that applies to faculty and students of Art & Art History is:

descreen for projection but do not descreen for printing.

If you scan an image from a book to reproduce in a paper or a poster (provided that you have the permissions that you need) the Moiré pattern will not reproduce when you print the picture again from a color printer. However, it will reproduce, loud and clear, on any presentation slide that you project, larger than life, on the big screen.