Scanning Original Material

If you are scanning slides or prints in order to create the high-res CMYK files, then, to determine the correct settings for the scanner, first figure out the percentage of enlargement or reduction you need (I use a formula based on “divide what you want by what you have” on a calculator), then double the desired halftone line-screen value to get the dpi setting. Then use that percentage and that dpi value as the scan settings for your image.

Incidentally, while scanners of amazing quality are now available at even more amazing prices, it is worth checking up on the accompanying software before buying, especially the kind of interface that is available if you want to scan directly into Photoshop. Some scanning software aims at the casual user rather than the professional, and will give you only a few presets, rather than the range of settings you need. If you cannot enter specific dpi and percentage scanning values, you would be better off with a different scanner, however tempting the deal appears to be.

8.6 Left: not descreened at all. Center: descreened at 90lpi. Right: descreened at 150lpi.

8.7 The RGB channels. The moiré pattern appears in the blue channel.

The CMYK channels. The moiré pattern appears in the yellow channel.