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We've received a request for this, so here it is:
A GUIDE TO RESOLUTION...
Part one: Common terms
One of the most potentially confusing things about working digitally
is the aspect of resolution. Much of the confusion can be cleared, however, by
familiarizing yourself with the relevant terms; what they mean and how
they relate to each other. Once you do understand the terms, it's really just a
mathematical process to achieve your desired result.
PPI: Pixels per inch
This is your display resolution, i.e., what you see on-screen.
Usually referring to a monitor or digital camera.
It also greatly affects file size and image efficiency - more
on that later.
This is the most often encountered resolution term, and it alone
can be confusing because the term relates not only to the on-screen display of any image, but also to the image file itself. To confound us even more, many people inaccurately use the
term "DPI" in its' place, but don't be fooled. They usually mean
"PPI" - "DPI" relates more accurately to printing an image
(the description of DPI comes a little later).
A side note: Whether you're on a Mac or PC does
have some bearing here, since Mac screen resolution is 72 ppi,
and PC screen resolution is 96 ppi. For PC users, exchange 96 ppi for 72 ppi
in the math, and you'll get the result for your system.
First, let's address the way this term relates to the on-screen
display of an image. In practical terms, a sample image
3" wide @ 150 ppi (file resolution) displayed on your monitor
(screen resolution) at 100% size, is 6.25 inches wide.
3 inches X 150 ppi (file resolution) = 450 pixels
450 pixels ÷ 72 ppi (screen resolution) = 6.25 inches.
Suppose the image was 3" wide at 300 ppi...
If you do the math, you'll see that the on-screen display would
be twice the width - 12.5 inches wide at 100%.
Note that both examples are 3" wide. The difference is
increased resolution. If you print either image, you'll get
an image 3" wide. The image with increased resolution will, however,
print at a much better quality on an average or better printer.
And that leads us to the next term...
DPI: Dots per inch
This is print resolution. Commonly referred to as density. This term
is often misused when actually describing ppi, even though a pixel and
a dot are not the same thing. Scanners will sometimes show a
rating expressed as SPI: samples per inch. This is roughly equivalent
to the dpi specification, although we are again talking pixel
representations - not dots.
If you look at a printer for your system, somewhere in the description
of the product there will be specifications on the printer's
resolution. Expressed as dpi, an average example might be:
Resolution: 600 X 600 dpi black,
600 X 300 dpi color.
The higher the quality of output available, the higher these
numbers will be. Of course, no matter how high the printer's resolution is,
an image file generated at too low a resolution will still print poorly.
Later we'll discuss good practices for generating and scanning images
LPI: Lines per inch
This is a halftoning resolution. LPI (also called linescreen) is most relevant
to commercial printing. For example, the most common linescreen for commercial
printing is 133 lpi.
When preparing an image for commercial printing, the surest way to get
exactly what you want in print quality is to discuss the image and its'
end use with your client, your printer or your service bureau. They will usually have a preferred set of specifications
for their processes. They'll likely consider whatever their
specifications are to be "standard," so the earlier in the
process you have this discussion, the better. Often, there will be
a full set of working procedures they recommend you follow, from the
accepted software use to the colorspace to the final file format.
They may specify bit depth, and for DTP use, they may even provide
you with a postscript printer description (ppd) or driver to
set up your document for their printer.
Save yourself some grief and cover this ground at the beginning.
Click here for
A GUIDE TO RESOLUTION...
Part two: Preparing your image.
Click here for a printer friendly version of the complete tutorial.
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