Re: Gamma equalising?

"Ken" <yellow@xxxxxxxxxxxxxxx> wrote in message
news:44ded599$0$22362$afc38c87@xxxxxxxxxxxxxxxxxxxxxxx
Please bear with me - I may not have the various terminologies correct,
but hopefully you will be able to understand what I am trying to
communicate.

I have Photoshop 7.0 on one of my computers and CS2 on another.

I am involved in the preparation of a newsletter in which we usually
include images. These images may be scans of text documents, scans of
photos or photos directly from digital cameras.

Although I work in colour, the printed output is in black & white and is
produced by a commercial printer.

The printer requests that I send him the 16 pages for each newsletter as a
PDF file. To this end he has provided me with the PDF profile for his
printing machine to use when saving the PDF.

I believe this is a color profile. Install it on your system in the
approrpriate folder, and use Photoshop's convert to profile command to
convert your photographs to this profile.

I have found that on some occasions two photos that might look quite good
when viewed separately on the monitor appear unsatisfactory when printed.

Mostly this seems to happen when the photos are on two pages that are
printed on the same face of the paper e.g. page 2 and page 15.

This is a dot gain issue. I'll guess the images were too dark. Use of the
pdf profile to convert your images should help with this, though there will
be variation in dot gain depending on the side of the ***.

My suspicions are the overall gammas for each of the images varies too
much from one another for the printer to cope with. As a result, they
either both appear substandard or one page will have good quality and the
other page printed on the same piece of paper will be bad.

If you're not converting your images using the profile, they will likely be
too dark.

I know how to use the info feature to establish the average gamma (K) of
a 5x5 pixel area. I would imagine that if I could select a larger
(variable size) area to average, I would be able to adjust the various
photos to be closer to each other in gamma. I can't find a way to do
this.

Curves. Find a midtone, and set it to a consistent value. Also be caureful
that your shadows and highlights - areas with detail - are set to 90 percent
and 10 percent, respectively. You may also have pure black and white
areas - for example your text scans.

In summary. Am I correct in my diagnosis of the cause of the problem; is
my approach to solving it correct; can you help me with the procedure for
achieving the desired result?

Converting your images to the pdf profile is probably the biggest missing
piece. Other thoughts: get a book by Dan Margulis, which deals well with
printing issues in Photoshop, and check out the comp.publish.prepress group,
where issues like this one are discussed routinely by people with press
experience.
--

Mike Russell
www.curvemeister.com/forum/


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