Since raster
images have a specific resolution (i.e., a specific number of pixels
per inch), scaling a raster image involves the distribution of available
pixels across the designated space. Image resolution subsequent
to scaling is referred to as effective resolution. If an image is
enlarged, unless additional pixels have been added by means of interpolation
(resampling), then accordingly the size of each pixel must be increased—consequently,
the enlarged image will have fewer pixels per inch (lower resolution).
Conversely, if an image is reduced, unless existing pixels have
been discarded (downsampling), the size of each pixel must be decreased
(higher resolution). Although scaling reduction generally is less
problematic regarding visible defects, unnecessary resolution can
contribute to excessive physical file size. Cropping the scanned
image to the size intended for print also will benefit toward the
reduction of physical file size. If an image will require scaling,
scanning resolution should be adjusted accordingly.
When placing and scaling
images, you are changing what is known as the image effective resolution.
Effective resolution is a calculation of the actual resolution factored
for the scaling performed in the page layout application.
The math works out so
that if you reduce the scale of an image in the page layout, you
increase the effective resolution. As you increase the amount of
scaling, you will decrease the effective resolution. The equation
is:
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Actual
Resolution / scale = effective resolution 
