CMYK separations: a correction

Dear Friends,
A while back (a month? two months?) I posted CMY separations from the
RGB file along with default CMYK separations, for an image of three
calla lilies. The CMYK separations made no sense to me; I couldn't make
sense of them in terms of either the original image or the image I
wanted to print.

I now believe that these separations made no sense not just because I'm
used to looking at RGB separations, as I supposed, but because they
simply made no sense; in fact they were garbage separations, incorrect
separations that resulted from using a smaller image than Photoshop
could comfortably produce accurate CMYK separations for. I came to this
conclusion this morning when I decided to re-print all these calla
separations in a bigger size because I didn't think the very small
separations would express the subtle shading in the image well when
printed on gum. When I printed the separations larger (with the same
identical default CMYK settings as before) the separations came out
looking much more reasonable and sensible; a person could actually make
sense of them. I could even make sense of them.
 
I like making test prints with small negatives because I can get the
information I need without expending a lot of time or materials. I can
easily produce small tricolors from small RGB separations (like 2x3
inches) that are reasonably accurate representations of what a larger
print of the same image would look like. Apprently one cannot do the
same with CMYK separations. I'm not sure where the cutoff is for
accurate separations, but it seems to be somewhat dependent first on
image size, then on resolution, but also on the level of detail in the
image.

For example, with this calla image, any resolution from 72 to 1600 gives
me bad separations if the file is as small as 4 inches. When I made
6-inch separations from a 720 ppi file, the separations were what I have
come to believe today are the right default separations for this file.
But when I changed the resolution to 360 ppi for the 6-inch image, I got
the wrong separations again. To complicate matters further, a 4-inch
file at 360 ppi printed what looked, and printed, like accurate
separations when the image consisted of five simple color patches with
no detail in the color patches. So as I said, it seems to be dependent
on image size, image resolution, and level of detail all together, how
small an image you can get accurate default CMYK separations for.

I've posted the correct CMYK separations along with the incorrect ones;
I will soon remove the incorrect ones from the page, which will
eventually be incorporated as a permanent page on my website, but it
seems right to keep them there for the moment so you can see what can
happen if you try to generate separations outside Photoshop's apparent
range of size and resolution for files it can comfortably generate
accurate separations for. The separations are clear at the bottom of the
page.

http://www.pacifier.com/~kthayer/html/CMYK.html

Katharine Thayer
Received on Fri Dec 17 15:14:16 2004