Re: Determining tonal percentage?
Re: Determining tonal percentage?
- Subject: Re: Determining tonal percentage?
- From: bruce fraser <email@hidden>
- Date: Thu, 24 Apr 2003 12:29:13 -0700
At 11:44 AM -0700 4/24/03, Tyler Boley wrote:
I keep running into the same wall, using density measurements to
linearize monochromatic printing with quadtone inks in Epsons. There
seems to be a disconnect between the prepress/color management world
and the photography world with regard to numeric representation of
density percentage on paper. I realize dot gain, dot area, etc. are
very specific terms that I may be using erroneously.
All the tools I've tried, the Spectrocam dot gain software, the
Murray-Davies spreadsheet, and Bruce Lindbloom's density calculator
all seem to agree with each other more or less, so I assume they all
adhere to the same standards. But they all give (for an approximate
example) a reading of about 71% for a value I (a photographer) would
call approximately 50% on paper. Conversely, working in the opposite
direction, if I use these tools to tell me which of my printed ink
patches to map to 50%, it looks to me to be around 32% gray on paper.
I get something much closer when I enter a value of around 2.2
(coincidence?) for the mysterious N factor in Murray-Davies. What is
the N factor, and why is there a purely arbitrary factor in a
calculation used for such a specific and precise purpose? And where
in the world did Kodak come up with 18% as middle gray? Obviously my
lack of knowledge is screaming out loud here, and those last
questions are merely interesting and not to the real point.
Murray-Davies gets me close, but the arbitrary N factor leads me to
think it's still too subjective. I'd prefer to nail this.
Are there mathematical standards I can use to determine, knowing my
ink dmax and paper white, to use measurement data to linearize
monochromatic output by photographic standards? There must be some
connection, in Photoshop (for example) 50% gray is "middle" gray.
There is obviously a critical hole in my knowledge, or a basic
mistake
Thank you so much.
Tyler
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Density and reflectance are two different things, and as a
photographer, you're probably more used to thinking in terms of
reflectance than density.
The human eye doesn't respond linearly to changes in reflectance. The
Kodak gray card reflects 18% of incoming light, and our eyes perceive
that as a middle gray. It's NOT an 18% gray the way prepress thinks
of an 18% gray, it's an 18% reflectance gray. The nonlinearity of the
eye's response to reflectance can be approximated fairly well by a
power curve with a gamma of 2.2...
I know I'm raising questions rather than answering them. That's
because the answers are long, I'm not totally qualified to give them,
and you probably need to refine the questions anyway. But I suggest
you look at linearizing reflectance rather than linearizing density.
Bruce
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