Converting an analog image–real life, film-based prints or transparencies,
original artwork done in a natural medium–to a digital file format is
considered an art and only in recent years has digital camera and scanning
technology and equipment been available to consumers. Following are some
very basic guidelines for capturing a reproduction-quality image.
Is the resolution appropriate?
When taking digital photos or scanning images, a higher pixels per inch (ppi)
setting does not necessarily improve final output quality. Higher ppi readings
do, however, take up more storage space. For jobs that will be printed,
you’ll obtain the best results if you shoot/scan at twice as many ppi as
there are lpi (lines per inch, or line screen) in the final output.
For example, most of our printing is done at 150 lpi so you'll want to
shoot or scan the image at 300dpi. If you are planning to resample (enlarge)
an image for another use, you’ll need to capture it at a higher ppi by multiplying
your normal input (twice the lpi) by the percentage of the enlargement. For
example, an image to be printed at 150 lpi and with a 150% enlargement works
out like this: 150 x 2 x 1.5 = 450 ppi resolution. It is easier to reduce
than to enlarge captured images. Pixels can always be thrown away when image
size is reduced, but true image pixels cannot be added to maintain resolution
when an image is enlarged. Also, to prevent aliasing, type and linework need
higher input resolutions than continuous tone photographic images.
That relationship will guarantee that you capture enough color information
to accurately produce the halftone image. To get the best possible images
at the lowest cost in file size and output processing time, plan your page
and image sizes before shooting/scanning.
What are ppi, lpi, and dpi? And why should you care?
Together, ppi, lpi, and dpi represent a confusing trio of acronyms used throughout
the printing industry. Defined below, each actually refers to a distinct
measure of a separate step in the process of capturing and printing images.
ppi, or pixels per inch, is a measure of the amount of information scanned
in, or sampled, from an image. The finer the optics of a scanner, the higher
the scan resolution (degree of detail), which is critical to image quality.
lpi, or lines per inch, or line screen, is a measure of the frequency of
the halftone screen used late in the process to print an image. A specific
lpi is recommended for the paper stock being used. (Refer to the Printing
Guidelines Chart, located at the front of this document, which shows the
relationship of lpi to various paper stocks.) Working backward, the lpi that
you select in consultation with your printer determines what ppi is best
to use early in the process, when an image is scanned.
dpi, or dots per inch, is a measure of resolution of output devices such
as laser printers, copiers, digital proofing devices, imagesetters and platesetters.
