I just scanned the same picture twice. Once with Descreen off but the Unsharp Mask on, the second with Descreen on and no Unsharp Mask (won’t let
me pick both at the same time- I assume that’s standard?)
Yes, that is standard.
Descreen is used to scan an image that has been screened, which means printed with a pattern of dots. A printing press is not capable of printing any shades of color or gray; shades of color or gray are "faked" using a "halftone screen," which is a pattern of dots spaced close together. (Look at a newspaper with a magnifying glass and you’ll see.)
Anything printed on a printing press–including posters, newspapers, magazines, brochures, and anything like that–should be scanned with "descreen" on. Otherwise, you end up with a pattern of dots rather than a picture. Anything that is truly photographic, like a photographic print or a negative, should be scanned with descreen off.
Unsharp mask makes a picture sharper. if you turn on unsharp masking with a screened image, the pattern of dots is sharper and more exaggerated–exactly the opposite of what descreening does.
The file with
Descreen on is 1.25 MB, while the one with it off is 2.5 MB.
How did you save the file?
The size of the file depends on the resolution and size of the scan. Descreen on or off will not affect this in the slightest. However, if by "size of file" you mean "I saved a JPEG," then there might be a difference in the size of the JPEG, because a JPEG file is compressed, and the amount of compression depends partly on how many hard edges are in the image.
But JPEG compression degrades the quality of the image, and JPEG is only useful where file size on disk is important and image quality is not. So of course you would not save a scan you just made as a JPEG, right? Not unless it absolutely had to be a JPEG and no other format would work–for example, for the Web.
Is there a general consensus on the effectiveness of Descreen?
Most scanner’s descreen function works well. It depends on how good the scanner’s software is.
Is there a
better way to fix the moiré (or whatever it’s called – sorry, I’m a novice) using other software?
If the morie is being caused by the halftone screen, the best way to fix it is with the descreen function–the scanner will remove the moire as the image is being scanned, while it’s still in its most raw state. Descreening after the scanner is done makes for a picture whose quality is not as good, because the scanner has more information to work with.
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