For DTP
People (and other brave adventurers
into electronic print..) 
IMAGE QUALITY -- AN EXPLANATION
I would like to give credit for this, but I found it on a
client's bulletin board. They were kind enough to allow me to
make a copy. I knew the formula, but this explanation is much
better than the one I usually came up with. I hope that you find
it useful too!
With the exceptions of CD ROM photo catalogs -- whose images are
intended for layout position only rather than for reproduction --
almost every CD ROM collection of digital photos is promoted as
having high-resolution scans. But if you compare file sizes,
you'll realize that's NOT the case.
- A truly high-resolution digital photo, one that you'd
find in Sports Illustrated, for example, can be
anywhere from 12MB (for a 4-by-5-inch image) to 70MB (for
a two-page-spread photo). The CD ROM photo collections
currently on the market have files ranging in size from
500K to 22MB.
- To determine if the vendor's definition of
high-resolution is sufficient for your purposes, you need
to know if the resolution of the scans on the CD ROM and
the line screen to be used in printing your publication.
- Image resolution, the amount of
information in a digital file, is measured in dots
per inch. Screen frequency, the
density of the lines of dots used to print a
continuous-tone photograph, is measured in lines per
inch. Newspapers typically use a coarse line screen
of 85 lpi; in-house publications, 100 to 120 lpi; and
glossy magazines & good quality offset lithography,
133 to 155 lpi.
In general, optimal image resolution is
twice the line screen: optimal dpi = 2 X lpi.
But size is an important factor, too. When you reduce an
image, you increase the effective resolution; when you
enlarge the image, you decrease the resolution. So if
you're resizing the original scan for use in your
publication, the formula to use is :
dpi = 2 X lpi X
percent of the original size
IMPORTANT
- when dealing with huge
files ( 50 - 100 Mb), you can reduce the "resolution
to line screen" ratio to 1.5 without very noticeable
degradation of quality. This reduction
can have significant effect on final file sizes.
- Consider a CD ROM image that's 300 dpi and 5 by 7 inches.
You plan to print it at 65 percent of the original size,
using a line screen of 133 lpi. In this case, the optimal
resolution is 173 dpi (2 X 133 X 0.65), and the 300-dpi
original provides more resolution than you need to
successfully print the image.
- If the CD ROM image is only 72 dpi, however, you do
not have enough information in the file for good
reproduction (remember that the optimal resolution is 173
dpi). The printed image is likely to lose detail, with
some jagged edges where there should be smooth curves, or
to appear posterized, with rough graduations in tone and
unacceptable levels of contrast.
- This discussion has centered on CD ROM images, but this
formula developed here is a great tool in deciding the
resolution of any scan you make for use in a printed
piece. In the case above, if you had several images in
the piece -- or many more in a larger book -- scanning at
300 dpi (where 173 is all that is needed) would result in
file sizes that would be much larger that
necessary,wasting time on loading the files and precious
disk space.
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Written by Julian A. Foster, Jr. --- Last
Updated: 28 Dec 1996 11:18 AM