A reliable desktop flatbed scanner can be purchased for between $1000
and $2000. With a little practice, you can use your scanner to
produce excellent black and white halftones. You should not, however,
regard your desktop scanner as an alternative for a high-end, color
drum scanner. Although maximum resolutions on most desktop scanners
are high enough for optimal output to a high-resolution imagesetter,
higher resolution does not necessarily mean better image quality.
Most desktop scanners do not have the dynamic range (the ability
to capture subtle details and differentiate between a broad range
of gray shades and colors) of high-end scanners. However, if you
intend to alter your color images in a retouching program such
as Photoshop, and accuracy of color is not important, desktop scanners
are excellent tools. Other limitations of the desktop scanner as
a design tool are encountered when scanning line art.
Resolution
Images scanned at too high or too low a resolution cause reproduction
problems, such as softening of the image or pixelization.
Scanning
Line Art
The formula for scanning line art is: scanning dpi should equal
the dpi of the final output device. The maximum dpi of 600 offered
by most desktop scanners is fine for 600 dpi laser output, but
is inadequate for high-resolution output (Pride images films
at 3200 dpi). When scanning or digitizing line art, straight
or curved lines are created by a pattern of small square pixels.
The higher the resolution, the finer the grid; and the smaller
the dots, the smoother the edge.
Scanning Photos
Photographic images should be scanned to a percentage equal to
the size of the final output at a dpi two times the lpi (line
screen ruling). At Pride, our default screen ruling is 150 lpi
when printing on uncoated paper, and 175 lpi when printing on
coated paper. Therefore, images should be scanned at a resolution
between 300 and 350 dpi. Be aware, however, that the resolution
of the scan proportionately increases or decreases when scaling
the photo.
An image scanned at 350 dpi, and subsequently enlarged
200%, yields a final dpi of 175, which is inadequate for optimal
output at a line screen of 175. The output will appear coarse or
pixelized. Conversely, the same image reduced 50% increases the dpi
to 700. This results in data overkill and won't enhance image quality.
The resulting excessive data may actually soften the image. This
extra data also quadruples your file size.
It's important to remember that doubling resolution
increases file size by a factor of four. For scanning photographic
images, use the following formula:
2 x lpi x percentage of scaling = Image Resolution. Contact your
sales representative to confirm the lpi you should use as the
basis of your calculations.
CMYK vs. RGB
Desktop scanners read color images in RGB mode. RGB (red, green,
blue) is a system based on additive colors, which is used for
devices that radiate light, such as CRT monitors. CMYK (cyan,
magenta, yellow and black) is a system based on subtractive colors,
and works with reflected light from surfaces, such as paper.
To offset print images, desktop-generated scans must first be
converted to CMYK mode and separated into the four process colors.
The gamut of a color system is the range of color the system
is capable of displaying or printing. RGB has a much broader
range than CMYK, although RGB is still limited in comparison
to what the human eye can discern.
Remember, with the limited dynamic range of most
desktop scanners, not all colors are being captured to begin with.
What you see on your screen is not what you will get.
Realizing the color disparity between modes will
help you keep perspective when color correcting your RGB file for
conversion to CMYK format. While it is faster to make color corrections
in RGB mode, it may be necessary to make additional tonal adjustments
after you convert the image to CMYK, due to the loss of color information
in the translation from RGB to CMYK.
TIFF vs. EPS vs. DCS
Understanding when and why you should save a scan in a particular
format is important for accurate and appropriate output.
TIFF (Tagged Image File Format) files are slightly
smaller in size than EPS (Encapsulated PostScript) files, but they
add to the size of your layout document. A 200K Quark file with
an imported 2 MB TIFF file increases the Quark file size to 2.2
MB. EPS files, on the other hand, don't add to the document's file
size. When outputting to a laser printer or imagesetter, documents
containing EPS files will clear the screen and print faster.
If you intend to place your grayscale or line art
scans into a layout program and assign them to print in another
color, you must save them as TIFFs. EPS files are already saved
as PostScript codes and cannot be altered in a layout program.
Save a file as an EPS if you plan to create a silhouette
in Photoshop. EPS is the only file format that enables you to save
the clipping path of your silhouette.
If you would like Pride to handle you high-end scanning,
but do not have the memory capacity to handle these large files,
request ".e" low-res placement file.
When you place the low-res ".e" image in
your layout, the low-res file will automatically be replaced with
the high-res final image during final output. This high-res image
will be cropped, sized and positioned according to the low-res "place
holder." Caution ".e" file users: limit your cropping,
sizing, and rotating, since automatic replacement isn't a flawless
procedure. Also, be sure not to rename the low-res master file
provided to you. Doing so breaks the link to the high-res image,
which prevents the processor from locating and replacing the low-res
version with the high-res files.
Summary
- Follow the formulas of resolution and
scan the image at the final output size.
- Make all subsequent
manipulations in a photo retouching program such as Photoshop,
before you import the image into a page layout program.
- Using
your page layout program to size, crop, or rotate your scanned
images may not only lead to undesired results -
it can also crash a raster image processor (RIP) and delay
production.
- Resizing a scan has
a direct affect on resolution. If you bring your images
into Quark and then enlarge the scan, you may end up with a pixelized
photo. Photoshop enables you to resize an image while maintaining
resolution. Keep a record of your scans and their manipulation.
Request a Scanning Documentation Form from your sales representative.
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