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Resolution explained ...

Many scanners can scan images at a variety of resolutions. The scanner software typically can perform advanced functions such as enlarging the image (in addition to quality functions such as removing colour casts). You may feel that you should use the highest resolution for every scan to make sure you get the highest quality image for reproduction. After you make a few scans at the highest resolution, you'll realize two things:

• High-res scans take up a lot of disk space.
• High-res scans take longer to send to the image setter, and take longer for the image setter to RIP.

So what resolution do you need for your originals? The answer depends on how you plan to reproduce the image, and the ratio of the original image's size to the final output size.

For example, if you plan on displaying the photograph on a web page, you only need about 72 dpi resolution, using anything higher produces unusable data. If you plan to print the image on newsprint at 1016 dpi, you might need more data. If you plan to use the image in a magazine that outputs full colour at 3048 dpi, you'll need a lot higher resolution than the other examples.

You can use this formula to determine the scanning resolution for any original:

Scanning Resolution = LPI x Sampling Ratio x Enlargement Ratio

This formula works for all types of originals: 35mm slides, transparencies, and reflective art, whether continuous tone colour or black-and-white.

Scanning Resolution In general, it's better to have too much data than too little data. Most image editing applications allow you to resample an image. Resampling means that the software reduces the resolution of the image, which also results in reducing the file size. You can always reduce an image's resolution later. Although you can increase an image's resolution, you won't gain any detail unless you rescan the original.

Should you always scan at the exact resolution that gives you the proper data ratio for the final image? Not necessarily. If you typically reuse images in a variety of sizes, you should scan for the largest size and line screen, and then resample for each size or crop instance.

Line Screens LPI (lines per inch) is the line screen you plan to use when image setting the image. This value depends on the resolution of the image setter (image setter resolution is a completely different issue than scanning resolution), and how the job will be printed. For example, jobs printed on newsprint might need an 85 lpi screen, while jobs printed on glossy paper might need a 200 lpi screen.

Sampling Ratio The sampling ratio is the rate at which the scanner samples data from the image, with 1.0 being a one-to-one sampling. To make sure the scanner reads enough data, use a Sampling Ratio from 1.25 to 2.0, depending on your quality requirements. Usually 1.5 provides enough data for most commercial work. If the image is complex, you might want to sample at 2.0. If your line screen is going to be extremely high (250-300 lpi), you can sample at 1.25 to reduce the size of the scan file.

Enlargement Ratio The enlargement ratio is how much bigger or smaller the printed piece will be as compared to the original. If the final image is the exact same size as your scanned image, use a 1 in this part of the formula. If your final image is twice as big, use a 2, and so on. If you plan on reducing the size of the image, use a fraction. For example, to reduce the size of an original by 90%, you would use 0.9 in this part of the formula.