Scanning
Resolution
Resolution
is
measured in dots per inch (dpi). The higher the resolution,
the more dots per inch resulting in larger image files.
Although a high-resolution image gives more detail, the file
size may be so large that it is unusable. More importantly,
if the image resolution exceeds the resolution of the output
device (printer, slide printer, computer monitor, etc) then
the extra resolution is just wasted disk space. For images
meant for web pages, large files mean long download times.
The table below shows the resolution of some available
output devices. See the sections below on Scanning for Slide
Printers, Scanning for Posters, and Scanning for the Web for
appropriate settings.
Device
Resolution
(horizontal x vertical in dpi)
GGCS
LaserJet 5M
600x600
Epson
Stylus Color
1440x720
HP
Plotter (poster plotter)
300x300
color (max. of 200x200 recommended for posters),
600x600 black & white
Slide
Printers
not
more than 600x600
Monitors
72x72
commonly
Scanning
for
Slide Printers
When
scanning images that will be eventually sent to the slide
printer, it is important to remember that the finished slide
will be much smaller than the image you work with on the
computer. Excessive detail will be lost or make the slide
cluttered. Also, extremely large files (due to
high-resolution images) take much longer to print, so use
the minimum resolution necessary. Usually a 600x600 or at
most 800x800 dpi image will suffice.
Scanning
for
Posters
Most
people reading a poster will be standing 3 or more feet from
the poster. Therefore, any images must be large enough to
see, often more than 6 inches on a side. Such large image
dimensions would result in enormous files sizes if also high
resolution. Therefore, it is best that your final image be
no more than 600x600 dpi (the poster plotter only achieves
that for black & white images) at most. A 200x200 dpi
(or even 150x150 dpi) image is best for color plots.
It is important to remember that when you enlarge your
scanned image that the resolution will change unless you
resample the image at the same time. For example, if you
double the dimensions of a 2" x 2" 600x600 dpi image in
Photoshop using the default settings, the final image will
be 4" x 4" but 300x300 dpi. The number of pixels is the same
for both images, but the pixels have been enlarged. You can
preserve the pixel size by setting the resolution of the
enlarged image. However, it is important to remember that
doubling the dimensions of an image while maintaining the
same resolution will increase the image file size by a
factor of 4, so a 1.2 MB file will become an 8.8 MB
file.
Scanning
for
the Web
Most
computer monitors are approximately 72 dpi. Therefore, any
images with greater resolution are a waste of download time.
To obtain the correct image size on a web page, alter the
image dimensions, not the image resolution. The only reason
to include high-resolution images would be to make special
copies of images available for download to the user's disk
for later use.
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© 2001 UW Geology & Geophysics Computer Services |