File size is not a very good indication of whether the resolution of a
photo file is good enough. You can easily change the file size by a
factor of 10 by JPEG compression that makes no visible difference in
the photograph and does not change the resolution.
The printer that I think the Texas Caver uses (I steered it to them)
prints halftones at 150 DPI. The rule of thumb used to be that you
wanted to start with 150% of that resolution, which would be 225 DPI.
The printer asks for 300 for grayscale or color illustrations and 600
for black-and-white line copy such as cave maps. What I usually ask
people to send for the Association for Mexican Cave Studies is photos
that are at least 1200 pixels high or wide. The largest I usually
print photos is 4.44 inches, so that is good enough, even when I do
some cropping. If I want to use a photo on the cover, I go back and
ask for more.
Actually, it takes a powerful magnifier to see a difference in the
printed results, even if a map starts out at 300 DPI or a photo at
150. Where you really get in trouble is something that is just screen
resolution. Beware of photo software with one-click methods for e-
mailing or posting photos. Those typically automatically reduce the
resolution (and file size) on the assumption that the result is to be
viewed on-screen. E-mail the actual original photo file as an
attachment. Even with the highest quality JPEG compression, it won't
be bigger than 10 MB. (Do _not_ JPEG line copy such as cave maps. ZIP
or LZW compression is OK. As there is no such thing as a black-and-
white JPEG, this question doesn't arise, unless you've used grays or
colors in the map.)
Jill, the printer's prepress people will fuss if illustrations are
less than their recommended resolution or color illustrations are not
CMYK. Save yourself an exchange of e-mails and include in the
instructions with the PDF files something like "I know some
illustrations are less than the recommended resolution. They are the
best available. Don't worry about it. If I have forgotten to make a
color photo CMYK, don't worry about that, either." There is, of
course, no point in your upsampling (or downsampling) things just to
keep them happy. You probably already know that you have to be careful
making PDFs. Acrobat by default JPEG-compresses or downsamples things,
unless you tell it not to.
--Mixon
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