In order to produce high-quality images, it is important
to understand how the pixel data of images is measured and
Pixel dimensions The number of pixels along
the height and width of a bitmap image. The
display size of an image on-screen is determined by the
pixel dimensions of the image plus the size and setting of
For example, a 15-inch monitor typically displays 800
pixels horizontally and 600 vertically. An image with
dimensions of 800 pixels by 600 pixels would fill this
small screen. On a larger monitor with an 800-by-600-pixel
setting, the same image (with 800-by-600-pixel dimensions)
would still fill the screen, but each pixel would appear
larger. Changing the setting of this larger monitor to
1024-by-768 pixels would display the image at a smaller
size, occupying only part of the screen.
When preparing an image for online display
(for example, a Web page that will be viewed on a
variety of monitors), pixel dimensions become especially
important. Because your image may be viewed on a 15-inch
monitor, you may want to limit the size of your image
to 800-by-600 pixels to allow room for the Web browser
How large an image appears on-screen depends on a
combination of factors--the pixel dimensions of the image,
the monitor size, and the monitor resolution setting. The
examples above show a 620-by-400-pixel image displayed on
monitors of various sizes and resolutions.
number of pixels displayed per unit of printed length in an
image, usually measured in pixels per inch (ppi). In
Photoshop, you can change the resolution of an image;
in ImageReady, the resolution of an image is always 72
ppi. This is because the ImageReady application is tailored
to creating images for online media, not print media.
In Photoshop, image resolution and pixel dimensions 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.
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 of 90,000 pixels. Higher-resolution images usually
reproduce more detail and subtler color transitions 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
Monitor resolution The number of pixels or dots
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 translated 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.
Printer resolution The number of ink dots per inch
(dpi) produced by all laser printers, including imagesetters.
Most desktop laser printers have a resolution of 600 dpi and
imagesetters have a resolution of 1200 dpi or higher. To
determine the appropriate resolution for your image when
printing to any laser printer, but especially to
imagesetters, see "screen frequency."
Ink jet printers produce a spray of ink, not actual dots;
however, most ink jet printers have an approximate
resolution of 300 to 600 dpi and produce good results when
printing images up to 150 ppi.
Screen frequency The number of printer dots or
halftone cells per inch used to print grayscale images or
color separations. Also known as screen ruling or line
screen, screen frequency is measured in lines per inch (lpi)--or
lines of cells per inch in a halftone screen.
The relationship between image resolution and screen
frequency determines the quality of detail in the printed
image. To produce a halftone image of the highest quality,
you generally use an image resolution that is from 1.5 to at
most 2 times the screen frequency. But with some images and
output devices, a lower resolution can produce good results.
To determine your printer's screen frequency, check your
printer documentation or consult your service provider.
and 600-dpi laser printers use screening technologies other
than halftoning. If you are printing an image on a
nonhalftone printer, consult your service provider or your
printer documentation for the recommended image resolutions.
File size The digital size of an image, measured in
kilobytes (K), megabytes (MB), or gigabytes (GB). File size
is proportional to the pixel dimensions of the image. Images
with more pixels may produce more detail at a given printed
size, but they require more disk space to store and may be
slower to edit and print. For instance, a 1-by-1-inch,
200-ppi image contains four times as many pixels as a
1-by-1-inch, 100-ppi image and so has four times the file
size. Image resolution thus becomes a compromise between
image quality (capturing all the data you need) and file
Another factor that affects file size is file format--due
to varying compression methods used by GIF, JPEG, and PNG
file formats, file sizes can vary considerably for the same
pixel dimensions. Similarly, color bit-depth and the number
of layers and channels in an image affect file size.
Photoshop supports a maximum file size of 2 GB and
maximum pixel dimensions of 30,000 by 30,000 pixels per
image. This restriction places limits on the print size and
resolution available to an image.