Adobe PHOTOSHOP ELEMENTS Manual page 55

The number of pixels displayed
Image resolution
per unit of printed length in an image, usually
measured in pixels per inch (ppi). In Photoshop
Elements, you can change the resolution of an
image because image resolution and pixel dimen-
sions are interdependent.
The amount of detail in an image depends on its
pixel dimensions, while the image resolution
controls how much space the pixels are printed
over. For example, you can modify an image's
resolution without changing the actual pixel data
in the image—all you change is the printed size of
the image. However, if you want to maintain the
same output dimensions, changing the image's
resolution requires a change in the total number
of pixels.
72-ppi and 300-ppi images; inset zoom 200%
When printed, an image with a high resolution
contains more, and therefore smaller, pixels than
an image with a low resolution. For example,
a 1-by-1-inch image with a resolution of 72 ppi
contains a total of 5184 pixels (72 pixels wide x
72 pixels high = 5184). The same 1-by-1-inch
image with a resolution of 300 ppi contains a total
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of 90,000 pixels. Higher resolution images usually
reproduce more detail and subtler color transi-
tions than lower resolution images. However,
increasing the resolution of a low-resolution
image only spreads the original pixel information
across a greater number of pixels; it rarely
improves image quality.
Using too low a resolution for a printed image
results in pixelation—output with large, coarse-
looking pixels. Using too high a resolution (pixels
smaller than the output device can produce)
increases the file size and slows the printing of the
image; furthermore, the device will be unable to
reproduce the extra detail provided by the higher
resolution image.
The number of pixels or dots
Monitor resolution
displayed per unit of length on the monitor,
usually measured in dots per inch (dpi). Monitor
resolution depends on the size of the monitor plus
its pixel setting. Most new monitors have a
resolution of about 96 dpi, while older Mac OS
monitors have a resolution of 72 dpi.
Understanding monitor resolution helps explain
why the display size of an image on-screen often
differs from its printed size. Image pixels are trans-
lated directly into monitor pixels. This means that
when the image resolution is higher than the
monitor resolution, the image appears larger
on-screen than its specified print dimensions.
For example, when you display a 1-by-1 inch,
144-ppi image on a 72-dpi monitor, it appears in a
2-by-2 inch area on-screen. Because the monitor
can display only 72 pixels per inch, it needs
2 inches to display the 144 pixels that make up one
edge of the image.
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