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Topic: lighting conditions, CMYK, Photographic processes and absolutes (130 lines)
1) From: Kevin DuPre
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I AGREE - comparing SCAA coffee color samples to your
coffee is NOT the same as a PICTURE of coffee to
coffee - too many variations in ink colors, printing
processes, etc. - I do understand the physics of this
and agree with the above statement.
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The Agtron machine is electronic instrumentation with
transducers, NOT your eyes. As such the transducers
are sensitive to a particular wavelength of light and
are calibrated to produce a correct reading at that
wavelength. I also have a background in the design of
electronic and computer controlled instrumentation and
am well aware of the issues with transducer design and
calibration.  Near infrared is a stable light source
over time - the same cannot be said of incandescent or
flourescent light sources using a gas or filament
under electrical stimulation to produce the light -
they change over time.
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Not a valid experiment, due to red-shift from the
angle the sun comes through the atmosphere, the amount
of sulphuric acid droplets and other chemical
constituents in that atmosphere, etc. All film, both
B&W and Color has a characteristic curve of light
sensitivity to the emulsion.  That curve can be
approximated as linear over its middle and through a
fairly wide latitude of exposure, illuminated with
"high sun" daylight, or electronic flash.  Red-shift
occurs outside of approximately 10AM to 2PM. The other
lighting sources produce different color shifts, many
of which cannot be "matched out" due to the
sensitivity of the different emulsion layers in the
film and the compensation for daylight that is built
in. Additionally the C-41 color process even with
turnkey lab machines is not that repeatable for
instrumentation purposes. The chemicals change
throughout the course of the day, becoming heavier and
heavier with silver deposits with the more film
processed. The machine has calibrations that
compensate, but it is not exact for instrumentation
purposes - only your eyes - the intended recipient of
the photos, even the built-in color adjustments are
compared by the technician to a "best match" for what
"looks right".
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Color shifts in the film emulsions which are designed
for "high daylight" shooting as per above. While your
experiment does make sense to you, you are not taking
the designed color sensitivity and compensation for
"photographically correct" daylight sources into
account.
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The Agtron disks are manufactured under a different
process than the CMYK process used to print Kenneth
Davids book and 4 color process-printed books in
general.  Kenneth Davids warns against using these
color tiles in the back of the book for this reason
and due to other reasons that people have posted,
particularly metamerism.  I agree that if you are
comparing coffee to the color samples in the back of
the book (or any other printed photograph of "coffee
at a particular stage of roast") you will see
different color shifts in different conditions. The
Agtron disks, since they are analagous to
instrumentation  for those who can't afford an Agtron
machine are more than likely produced with a paint
pigment which is photochemically stable and checked
with a precisely calibrated piece of instrumentation
against the Agtron color standard. It is this process
of calibration and precise production that results in
the high cost of production.
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CMYK is only a method of SYNTHESIZING real colors with
halftone screens with varying dot-densities of 4
specific monotone colors.  CMY can only produce color
and saturation. Using only CMY you cannot produce all
the colors of the spectrum in all their degrees of hue
and saturation. The black is needed to produce hue for
"dark" colors. Dot densities of less than 100% are
used to produce hues at the "light" end of the
spectrum. With each screen at 100% one would overlay
the other following 4-color process and you would have
the most recent color tinted by residual colors of the
other two, which is why halftone screens are used and
4 process colors are used - it's only an approximation
to appease your eyes, not an exact match that is of
instrumentation quality.
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I don't have those books so I can't say I agree or
disagree.
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I wholeheartedly agree with this statement - it's the
basis for color photography and for the effects
acheived in B&W photography.
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This statement doesn't make sense - how can you have a
"mix" of "one frequency color"?
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The point I was trying to make is that using Agtron
color tiles to compare to your coffee using the
recommended procedure under a BRIGHT light will
produce the same RELATIVE PERCEPTION. It is your eyes
and brain making the judgement for all the reasons
that I have stated above.  If you shift that light 
into high amounts of red or blue, then it is true, you
will create a discernible difference. But I don't
think that anybody will be comparing Agtron color
tiles under those conditions.  My argument was from a
practical and common sense standpoint - how would I
use the Agtron tiles to measure my degree of roast?,
not how would I perceive the Agtron tiles under any
and all lighting conditions - the latter is highly impractical.
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--
Kevin DuPre
obxwindsurfhttp://profiles.yahoo.com/obxwindsurf"The real voyage of discovery consists not in seeking new landscapes but in having new eyes -- Marcel Proust"
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