Re: Projection Color-T

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Wed, 8 Oct 1997 17:28:03 -0500
>From: P3D  <Linnstaedt@xxxxxxx>
>Subject: Re: Projection Color-T

>Paul S. Boyer writes:
><< ...The worst effect is the overcast blue.  I have never found a really
> good way to correct for this, and find the best solution is simply to
> avoid shooting in those conditions.  Even filters don't help much,
> because if the warm colors are not there, they just aren't there.
> --Paul S. Boyer   <boyer@xxxxxxxxxxxxx> >>

>Though Prof. Boyer cannot get satisfactory results, this does not indicate
>impossibility.  The fact that many photographers are able to, suggests that
>Mr. Boyer shouldn't give up so easily.

>...Most problems with filters are due to insufficient light, since
>exposure has to increase for the filter's factor.  The only case where it's
>impossible is when you pre-view the scene through the filter and only see
>inky blackness!  Perhaps I am not understanding.  Fill me in?

>Robert Linnstaedt

The following thought experiment is based upon speculation:

Imagine a scene which is made up entirely of red objects (objects which
reflect only red light) and blue objects (reflect only blue light). Imagine
a a light source which combines spectrally pure red light and spectrally pure
blue light in equal proportions. Also imagine color film with only two layers,
one sensitive only to red light and one sensitive only to blue light.
If this combination (scene, light source, and film) is used to make a
photograph, both the red objects and the blue objects will be well represented
in the photograph.

Now suppose that the red light is eliminated entirely from the light source,
so that only blue light is present. An attempt to photograph the scene will
result in the blue objects appearing blue, and the red objects appearing
black. No amount of filtering will make the red objects appear anything
but black. That would appear to be pretty close to the situation Paul was
describing.

To make the model more applicable to a real-world situation, suppose that
instead of being eliminated entirely, the red content of the illumination
source is reduced to 1/32 of its original level - the red objects in the
photograph will not appear black, but they will appear very dark. To
attempt to get the proper color balance, I suppose you could introduce a
filter that has no effect on red, but reduces blue by a factor of 32,
then increase the exposure by a factor of 32 (or more, depending on the degree
of reciprocity failure) to compensate for the light loss. If the increased
exposure is impractical, that could be called an "insufficient light"
problem.

If you have more than two colors, the problem becomes much more complicated.
To produce a proper color balance, the filter must perform an "inverse
transform" of the unacceptable light source to the desired "normal" color
balance, across the spectrum. Looking through the filter, you should see
a normal color balance (though that's hard to judge at reduced light levels),
not just "not inky blackness". Even worse, the film spectral sensitivity is
only an approximation of that of the human eye, designed to produce results
that look more or less normal under certain assumed lighting conditions.
So it's really the film that you need to "fool" into "thinking" that the
color balance is "normal". That may not be impossible, but it may be
horribly difficult under some lighting conditions.

That being said, I've seen plenty of photos of the ocean, shot through an
orange filter, which people crowd around and comment on the beautiful
"sunset photo", and plenty of "nighttime" scenes in movies which were
pretty obviously shot in full sunlight, with reduced exposure to the film.
But my personal experience with shooting photos of autumn leaves is that
they need golden sunlight to appear at their most striking, and I haven't
come across any filter that will make them look just as good on a completely
overcast day.

John R


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