Re: Subject: Determining tonal percentage?
Re: Subject: Determining tonal percentage?
- Subject: Re: Subject: Determining tonal percentage?
- From: Jim Rich <email@hidden>
- Date: Thu, 24 Apr 2003 22:52:59 -0400
On 4/24/03 8:52 PM, "Gerry Yaeger" <email@hidden> wrote:
>
Adding to Bruce's fine answer or questions, I recall from the old days of
>
making halftones on a process camera we would generally set the midtone to a
>
density value of .65 to .75 on the Stouffer scale. That would support your
>
finding of a 50% value having a density of .71
>
As Gerry, pointed out in the old days you could create a halftone
reproduction using density on a grayscale wedge to guide you. In his
example he was referring to reflection density.
In color separation of transparencies, for example, you can also use density
to define the tone reproduction (Image contrast). This method is based on
measuring from the halftone target value that was placed in the highlight
density of the original. From the highlight if you measured higher
densities you can define the 1/4 tone, midtone and 3/4 tone areas. For
example if your highlight density is .30 then the 1/4 tone is measured at
.60. The midtone is measured .90 about the highlight, at 1.20 density. The
key is to know what halftone values that should be at those densities. Of
course the highlight and shadow are set before the midtone values are
measured.
Thinking back, I learned this in the 1970s by reading Kodak and Dupont tech
notes. When the 1/4 tone, midtone and 3/4 tones densities points have the
correct halftone values (output values) you get a good visual contrast
match to the original. Then if you get the gray balance correct (based on a
good tone reproduction) you then only have to deal with selective color
issues. And if you are lucky your scanner could help with that. Today a lot
of this is done automatically using profiles.
So one question might be, what are those magic halftone values?
In a traditional printing process on different papers the problem is that
the tone reproduction halftone target values change. But the news is not all
bad. There are some realistic guide lines that can help.
At one point in the 1980s I visited over 300 businesses and helped them set
up their color separation processes using cameras and drum scanners. In that
time I noted the tone reproduction values of those sites and found that in
over 80% of the sites, when a scanner was set up correctly the tone
reproduction target densities for highlight, 1/4 tone, midtone and 3/4 and
halftone values were within one or two percent for proofing systems like 3M
Matchprint or Cromalins that had a direct relationship to the press. Gray
balance values for those proofing systems were also with in two or three
percent.
Here are some guidelines for target densities and halftone dot values for a
tone reproduction values using a full color separation without any UCR. In a
lot of ways these values are very similar to the Photoshop profile
U.S.Sheetfed coated v2.
HL Density .30 Halftone value 5c,3m, 3y
1/4tone D .6 Halftone value 28c, 21m, 21y
Midtone D 1.2 Halftone value 61c, 50m,50y,10k
3/4 tone D 1.8 Halftone value 78c, 67m, 67y, 50k
Shadow D. 2.7 Halftone value 95c,85m, 85, 80k
And of course, if you are not a halftone dot person you could do some
testing to determine optimum RGB levels instead of the dot values to get a
good tone reproduction based on the target densities.
Jim Rich
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