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Adobe Photoshop can indeed save bitmapped images in EPS format. This format
gives you control over printing parameters such as the screen frequency,
screen angle, resolution (scaling) dot shape (round, elliptical, etc.) and
transfer (dot gain) functions. You can also create EPS images that contain
separation information for Cyan, Magenta, Yellow and Black. I often use EPS
for screen shots in my FrameMaker documents because Frame does not have the
elaborate prepress options found in Quark and PageMaker.
I once worked for a company that manufactured clinical MRI machines. The
manuals contained MRI images and I produced the manuals on the DocuTech 135.
The manuals were used for troubleshooting and calibrating the instrument and
showed the types of image artifacts that can occur when, for example, the
magnet isn't properly shimmed. In order to reproduce these images with the
best quality possible, I printed a test pattern with increasingly dark
shades ranging from 0 to 100 percent in five percent increments. I then
measured the results with a densitometer and built a calibration curve in
PhotoShop. I saved all MRI images in EPS format and included the calibration
curve and screen frequency settings. The resulting images had much more
linear grayscale reproduction than would be possible using TIFF images and
the DocuTech's uncalibrated defaults.
Having done a lot of prepress work in my college days, I confess to being
somewhat of a control freak when it comes to these parameters. I reproduce a
lot of manuals on the DocuTech 135. If you import a screen shot as a TIFF
into FrameMaker and then generate PostScript code to run on the DocuTech,
the screenshot will be reproduced using the DocuTech's default 85 line
screen. This is the maximum line screen that will produce 256 grayscale
gradations on a 600 dpi device. However, screenshots, once converted to
grayscale, typically only contain a dozen or so shades of gray.
Consequently, you can obtain a much higher quality screen shot by
reproducing the image with a 120 line screen. The EPS file format allows you
to encode the desired screen frequency in the image and override the
printer's default settings.
