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Flatbed Scanning Hints

A word on SHARPENING and DESCREENING
A Soft picture
soft
A Sharp picture
sharp
 Descreening
Detail Correction option windowWhen scanning from a pre-printed page, colour or B&W, a pattern shows in the scanned image that clashes with the halftone dots of the final output. If the scanning software offers the option to include Descreening while scanning then this option is worth a try. Some descreening algorithms work remarkably well although experimenting with different scanning resolutions will often bring about the best results.

If the decision has been made to scan at a resolution of 300ppi because the halftone output will be #150, then perhaps 450ppi or even 240ppi could be tried and the image interpolated in a painting program back to the desired 300ppi.

Major Photo-retouching programs have a similar option to Photoshop's Despeckle filter, and it would be desirable to compare the results with the scanning softwares Descreen option - in either case, some softening of the image will result - another consideration.

Sharpening
As most flatbed scanned images improve considerably after a general sharpening filter is used, this option should also be compared with the results of sharpening in a Photo-retouching program and not the scanning software. You have to find which is best with what you have got.

Note: If an image is to be retouched after it has been scanned, always leave general Sharpening and Softening options until the very end of the Photo-retouching session and do so ONLY while viewing the picture at a 1:1 zoom ratio.

Interpolation
Interpolation, even in it's most primitive form has to be performed by a flatbed scanner. There are no optics to enlarge or reduce the image that falls on the Photosensitive array that records the picture information, so if the image is to be made bigger, or smaller, then pixel information has to be either made up from other pixel information or simply removed and adjacent pixels altered (see separate tutorial page describing Interpolation).

A WORK AROUND!
A bad scanThis example may tweak the imagination when other scanning problems are encountered too.

An all too common problem with flatbed scanners is one where the tiny components that make up the photosensitive array are electrically unbalanced. These variations (minute values) in electrical current can cause even the smoothest of picture areas to appear as if they contain lines, becoming most obvious after an image has been sharpened. An enlarged section of a sharpened picture is shown where these offensive vertical lines are very obvious.

One way to reduce this affect and often remove it altogether from a Greyscale/Colour image is to scan the image at a higher resolution than required – 400 to 500 pixels/inch - and then resize the image in a Photo-retouching program down to the desired resolution, where the programs inbuilt interpolation (advanced pixel averaging) hopefully removes the pattern.

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