In an age of digital photos, there are still plenty of images that still need to be scanned from hard copy to electronic form. However, despite their push button ease of use, there are still a few things to keep in mind when scanning a photo or document for use in a desktop publishing project.
Resolution is the first potential trap found in image scanning. For a print project, don’t scan at 72 or 75 dpi (the default setting for some scanning software). This setting is good for an image to be used on a website, but it will not work in a print project; this resolution setting is too low, and your scan will be blurry and bitmapped. Conversely, if you have a scanner with 1200 dpi resolution, don’t fall into the trap of thinking that more is always better. If your print project is being output on a 300 dpi laser printer, anything over 300 dpi is overkill. If it is going to a printing press, the formula used for determining resolution is to take the line screen setting that will be used on the printing press (133 lpi is standard for most jobs, although lower line screens are used on printed material such as newspapers) and multiply it by two to get the dpi setting you need. Thus, a photo can be scanned at 266 dpi and safely used in an offset printed brochure.
The above settings assume that the visual is being scanned at the same size it will be used in your project. Blowing up or reducing the size of scanned images in your layout program changes the resulting printed resolution of your scan. If you scan something at 5 inches by 5 inches, for example (you can adjust these settings in your scanning software) at 266 dpi, and then decide to blow up the resulting electronic version of the image to twice that size in your layout, you will run into problems. The scan will in effect become a 133 dpi image, with all the resulting degradation in image quality.
Another issue that beginners and pros alike often find themselves dealing with is a moire pattern on scans of offset printed material (such as newspapers and magazines). This is a checkered pattern that visually detracts from the scan. One method of dealing with this is to use the built in descreening option available in many scanning programs.
Another way of dealing with it (still used by many desktop publishers) is to overscan the visual (for example, at 600 dpi when you only need 300 dpi), apply a Gaussian blur filter (available in Photoshop and other image editing programs) to the image (sample with the preview and only blur enough to get rid of the moire pattern) and then reduce the image and apply an Unsharp mask filter to reduce the unwanted blurring effects of the Gaussian blur filter.