Effective Resolution
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:
(Actual
Image Resolution) / (scale) = Effective Resolution
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Remember the effective resolution formula:
 Actual
Resolution / scale = effective resolution
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