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Resolution Basics: The End of "Resolution Confusion"
What Is "Resolution" and How Do I
Use It?
Bitmap images are composed of pixels. Image "resolution"
is simply the number of "Pixels Per Inch" (PPI) in the
bitmap grid. There are two aspects to every bitmap image - its size
(width and height in inches) and resolution (the number of pixels
per inch). These two factors alone determine the total number of
pixels in an image. For example, a 2 inch by 3 inch image with a
resolution of 300 pixels per inch contains (2 x 300) x (3 x 300)
or 540,000 pixels.
The more pixels there are in an image, the more detail the image
can be displayed with. The fewer pixels there are in an image, the
less detail the image can be displayed with. There are two ways
to display an image - on screen and in print. When you are preparing
images you need to know what resolution to scan or size them to.
There is an optimum resolution for each and it is very easy to determine.
First let us take up screen display. This is the easiest to determine.
Scan or size them all to 72 PPI. Why? A monitor on a Macintosh system
displays at 72 PPI. A monitor on a Windows system displays at 96
PPI. Though there is a difference between the two, the standard
for screen displays is 72 PPI. Piece of cake!
Continuous Tones
Determining image resolution for printed output
is a little bit different because of the way an image gets printed.
A typical photograph like a snapshot or 8 x 10 glossy of your favorite
movie star is known as a "continuous tone" photo. It is
called a continuous tone because of the gradual changes of tones
of color or shades of gray (for black and white photos).
Halftones
It is not practical to print continuous tones on
a printing press so a method was developed to simulate the changes
in tone using only black ink for black and white photos. For a photo
to be printed on a printing press it must first be converted into
a "halftone". A halftone is an image whose continuous
tones have been converted to a pattern of solid dots. When viewed
as a whole, this pattern of dots appears as a continuous tone, when,
actually, it is not.
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This grayscale image when printed is really composed of a pattern
of dots. |
Using a camera and film, you could take a continuous
tone positive and expose a negative with an ordinary window screen
positioned between the lens and the film and produce a halftone.
The light passing through the screen will diffuse and create the
halftone dots. Professional litho houses and printers have been
using a method similar to this, although not with a crude window
screen. With computer graphics it is done by the software and the
printer. You can create your own halftones on a laser printer to
get a feel for this.
A halftone is expressed in terms of the "screen frequency"
- the number of "Lines Per Inch" (LPI) of the screen used
to prepare it. The number of lines per inch is also referred to
as the "Line Screen" (LS) of the halftone. Halftones appearing
in a typical newspaper range from 65 to 85 line screen. These are
coarse halftones because the paper is very porous and rough and
requires a larger dot size than the smooth, coated stock used in
magazines or brochures. Halftones appearing in magazines, brochures
and high quality catalogs typically vary between 133 and 150
line screen.
When preparing images for printed output you must know the screen
frequency of the finished halftones before scanning or sizing them.
Find out from the printer what is required for the job. There are
a lot of variables you need to nail down ahead of time such as the
type of paper, the capability of the printing press, what kind of
printing plate will be used (paper or metal) and whether the plate
will be exposed from a shooting board positive or a film negative.
Once you know the screen frequency of the finished halftone, you
can scan and otherwise prepare your images. There is a rule of thumb
used in preparing images for printed output. Scan or size images
at a resolution (PPI) of 1.5 to 2 times the screen frequency (LPI)
of the finished halftone. For example, a 2 inch by 3 inch photo
would be scanned or sized to a 2 inch by 3 inch image at 225 to
300 PPI for a finished halftone at 150 LPI (150 line screen). Were
the same photo to be printed using a 120 line screen halftone,
it would be scanned or sized to 2 inches by 3 inches at 180 to 240
PPI. This is a very workable rule and is easy to remember.
You Determine The Halftone Frequency In The
Layout
Once you have prepared your images, you import
them into the page layout program such as QuarkXPress™, Adobe®
PageMaker®, Corel VENTURA®, Microsoft® Publisher or
whatever you are using. In the picture boxes or print dialog you
fill in the data fields telling the software what screen frequency
to print the halftones. The laser printer or high-end imagesetter
uses this information at output time and produces the halftones
on paper or film from the digital data. You use grayscale images
for black and white halftones and CMYK (not RGB) color images for
color separations.
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Images and text Copyright © 1998 by Mike Doughty, All
Rights Reserved. This tutorial reproduced here with permission.
For more tutorials and help, visit Mike's Sketchpad at http://www.sketchpad.net
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