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Re: [OM] Best of Both Worlds

Subject: Re: [OM] Best of Both Worlds
From: Ken Norton <ken@xxxxxxxxxxx>
Date: Mon, 15 Mar 2010 18:08:35 -0500
>
> are designed to mitigate this issue but I don't know that it would pass
> an AG purple  torture test  being able to render purple accurately
>

It's all in how the sensor "sees".  Not the processing.


> Taking Marnie with our purple clematis is my purple test.
> I'll be durned if I can get it perfectly right on digital with  reality
> directly present as comparison.


I'm not familiar with the reflection spectrograph of a purple clematis. It
may be possible that the red and blue portions of the flower fall within the
spectral response of most sensors. Some flowers are truly in the violet
range (sub blue) and unless the sensor's red pixels or film's
red-sensitivity layer have a secondary bump down below blue (the E-1 and
E-300/500 have this secondary bump), the camera only sees blue.

An African Violet, is a bit stressful on most imagers, both film and
digital. The color is not violet, but blue and red. But the red portion is
actually deeply into the cut filter of most sensors and beyond the spectral
response of many films.

So, the problem is two-fold:

1. A true purple may or may not be seen by a film or sensor if the UV cut is
too aggressive on the red-sensitivity layer or pixels

2. A purple made up of red and blue may or may not be seen by the film or
sensor if the IR cut is too aggressive on the red-sensitivity layer or
pixels.

But if you widen the UV and IR cut filters too much, then you get purple
blacks (Leica M8, Kodak 14n, Olympus E-1), or inconsistent colors between
different light sources. Flash will yield full-spectrum (and nasty purple
blacks if overexposed) and hold purples purple, as will direct sunlight. But
overcast or indoor lighting which blocks the IR and UV components of
sunlight will shift the colors to blue.

It is impossible to correct for these in editing unless you manually do a
color shift on each and every image in selected portions of the image. You
can't make visible that which isn't there--either the film/sensor saw it or
it didn't.

Just a note, here on this. If you take a photograph of a purple picture, the
end result will be a nice match of purple--no matter of what the original
picture was of. This is because in a photographic print, the dyes or
pigments are themselves within the spectral response of any film or sensor.
This is why photographic tests and profiles using printed charts or targets
are bogus. All you have managed to do is calibrate your profile to
photographing a set of pigments or dyes under artificial lighting. The
actual spectral response of the colors on the targets is usually anything
but like the natural world. Take human skin, for example--it has a
tremendous IR componant to it that our eyes and many films and sensors see
which gives the luminancy we identify as skin but is missing in the
photographic print. This, BTW, is also why slide-projection often looks more
"natural" than printed or digitized images--because the slide-projector is
also projecting UV and IR light which our eyes are seeing. When we see a
slide being projected and there is human skin visible, it will look
uncanningly real.

One "easy" way to determine the spectral reflection of a flower, view the
flower through a UV filtering lens. Most sunglasses do this. If the flower
turns black or near black, then the color truly is violet. If the flower
turns reddish, the flower is actually red and blue. If the color remains the
same, both red and blue fall within normal spectral response. With your B&W
filters, you can use the yellow filter to see if the flower turns black. If
it doesn't turn black, then it's mostly red/blue. If you use the red filter,
if the flower doesn't significantly darken then it is mostly red with
potentially a significant portion of near-IR.


AG (color me blue) Schnozz
-- 
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