Re: CMYK separations: a correction

Katherine,

I'm curious regarding why image resolution would affect color and density of
an image. I understand that image resolution would affect detail in an
image, with the detail being interpolated up or down, but otherwise the rendering
of the colors in the separations and relative densities should not change.
I'm not talking about RGB vs CMYK, but just the CMYK info you mentioned below.

Are you sure your Photoshop settings and Printer Driver settings were
identical in all cases? Order of inversion of seperations in the workflow all the
same?

With regards to PPI of the file sent to the printer driver, I've not seen
evidence that anything other than 360 is optimum.

I'm not doubting that you saw something different, just baffled that it could
possibly be resolution causing it—unless the file was so tiny and
interpolated up so far that so much data was lost that it would be impossible to render
the image—however if that was the case, you would easily see this on the
original image on the screen.

Best Wishes,
Mark Nelson
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www.MarkINelsonPhoto.com

In a message dated 12/17/04 3:15:10 PM, kthayer@pacifier.com writes:

> 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 18:11:42 2004