From Lidka's Articles:

Tips on Scanning for Print

1.Use the ppi formula to scan at the proper resolution:
ppi=1.5-2.5 x lpi x sizing % (sizing can be controlled through scanner software)
This can save you many megs of unnecessary information — the larger the image dimensions, the bigger the file size. Follow this handy rule: for images intended for output at less than 133 lpi, scan at a halftone factor of 2 x lpi; for images which will be output at 133 lpi or higher, use 1.5 x lpi.

2. Scan line art to 800-900 ppi maximum, using the formula:
ppi = dpi (times sizing factor, if not calculated by a scanner)

3. 16-bit (per channel) color is a waste of RAM and hard disk space—don’t use it. Stick to24-bit color (8-bit per channel). Unless you’re scanning from high-density transparencies, you won’t be able to see the difference.

4. Good descreening control is a must, so be sure you’re using a scanner with descreening software. When you scan from offset print originals, you superimpose a grid of pixels on top of halftone dots, producing a crosshatch effect (called moiré) which is particularly visible on flat color areas (e.g.human skin) and is very difficult to eliminate.

5. Resize originals to the final output dimensions using scanner software.

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