For printed images, the resolution is based on the size of
the dots that make up the tonal values of the images. It is usually expressed
in dpi (dots per inch). The higher the dpi, the higher the resolution and the
higher the quality.
In scanning, optical resolution refers to the amount of detail captured by
the scanner. The optical resolution is expressed as two numbers, such as 600
x 1200 ppi (pixels per inch). The first number is the quantity of pixels per
inch that the scanner captures in the horizontal direction. The second number
is the quantity of steps passed over as the scanner head moves in a vertical
direction. The actual resolution of the image is only the number of pixels per
inch, not the number of steps. Some scanner resolutions are referred to as interpolated
or enhanced resolution.
Generally, images should be scanned at a resolution of twice
the line screen (lines per inch or lpi) that you plan to use for printing, unless
you plan on enlarging or reducing the image.
Scanning Resolution = 2
x Screen Frequency (lpi)
If you are going to be enlarging the image, scan it at a higher
resolution and use the following formula:
Scanning Resolution =
Largest Sized Final
x 2 x
Screen Frequency (lpi)
Largest Sized Original Image
After you have scanned the image using the proper
resolution, go to the "Image Size" dialog box and enlarge your image,
with the "Resample Image" check box deselected. Your resolution will
then be at the proper setting.
The only exception to this rule is when black-and-white
line art images are being scanned. They should be scanned at a resolution which
is set as high as possible for the clearest possible reproduction. 1200 dpi
is usually the standard setting.
If you don't know which
line screen will be used, ask your printer. When asking the printer
about the resolution, let them know which kind of paper will be used. Generally,
newspapers are printed at 85 lpi, and magazines are printed using either 133
or 150 lpi.
Scanning at a higher resolution than you need
only consumes more memory than necessary and produces a larger sized file that
is harder to work with. If you scan at too low a resolution, your image may
be blurry or you may see the individual jagged pixel elements in the image.
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