Microtek ScanMaker 3600 User Manual page 107

Basic Scanning Concepts
This section covers basic scanning concepts. If you already have
basic scanning knowledge, you may skip this section.
What is a scanner
Type of scanners
Components of effective scanning
Image types
Text scanning
A rule of thumb for choosing the right Quality Factor is, for images
with printing screens less than or equal to 133 lines per inch, set
Quality Factor to 2; above 133 lines per inch, set it to 1.5; contone
printer (continuous tone printer, such as dye-sublimation printer),
set it to 1.0
If you're outputting images to a monitor (such as doing multimedia
work), you need not scan images higher than 72 ppi, as monitors
are capable of only showing images up to 72 ppi. A higher-
resolution image will not be any clearer on the monitor and will
simply create larger files.
Remember that the higher the resolution, the larger your image file
will be. For instance, an 8.5" x 11" color photograph scanned at 75
ppi takes up about 1.6 megabytes (MB). Doubling resolution to 150
ppi will increase the file size four times - to approximately 6.3MB!
Going to 300 ppi will increase file size to 26.2MB.
What you need to do then is to select the lowest possible resolution
that still gives you good image quality in order to keep file sizes
manageable.