Re: polarizing filters with b&w infrared

[boblong@xxxxxxxxxxx (Robert Long)]

On Thu, 31 Oct 1996 04:43:36 +0000 (GMT), JoePaduano wrote:

|Bob-- check my reply to Tom regarding the black skies and haze reduction=
 that
|occur with the use of only a red filter, no polarizer. This is to the
|absorbtion of blue light by the red filter. A polarizer will not block =
the
|blue light. I f skies are rendered black with the red or infrared =
filter, how
|much darker can they be with a polarizer added?

A lot. it seems.  The effect is very dramatic looking through the SLR
viewfinder, but of course you're seeing only one portion (about
one-third??) of the "light" that will be creating the exposure.  So
you can't expect equally dramatic differences in the negatives.
Looking--not very carefully, I must admit--at the negatives before
sending them off for contact sheets, I thought I saw a significant
difference in sky density in some shots that I had deliberately made
with the polarizer set both for maximum and minimum sky darkening.
When the contacts come back, I'll post what I see.

Naturally, the effect varies with sun and camera orientation.  I was
working on a mostly clear fall afternoon with some high haze.  When I
shot toward the northwest, reorienting the polarizer made only a
modest difference; the dramatic differences all occurred when I was
shooting roughly northeast.  In shooting south I don't remember seeing
any difference at all.

And part of the equation is the darkness of denuded deciduous trees
vs. the brightness of those trees to IR when they're in leaf.  Closing
down the lens or printing darker to compensate for that brightness
will help keep skies very dark relative to the foliage, so perhaps no
difference whatever would then show up in the print when utterly clear
skies are involved.  But opening the lens--or printing lighter--once
the leaves are gone necessarily lightens the sky, all other things
being equal.

The point of confusion in our previous posts appears to be over the
filtration used in conjunction with the polarizer.  I was using a #25,
which admits maximum visible light; the Kodak booklet was showing
results with a #87, which admits little visible light.  I might also
point out that, far from saying that polarizers are useless with IR
films, the Kodak booklet says that results vary and goes on to
reference a case where, with unknown filtration, the combination
yields results that apparently are unavailable any other way.

I won't be doing another round of HIE until there is snow on the
ground.  But when I do, I'll try to devise some tests to check
specifically the difference between #25 with and without polarization.
In what I shot last weekend, I was going only for pictorial quality,
and the addition of a polarizer was essentially a whim.

Bob Long
(boblong@xxxxxxxxxxx)

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Topic No. 11