Re: polarizing filters with b&w infrared

[boblong@xxxxxxxxxxx (Robert Long)]

On Fri, 08 Nov 1996 01:20:36 +0000 (GMT), Scott Donkin wrote:

|So, for the pic you mention, which was shot with a Pola filter and #25,
|there WILL be some polarizing affects, simply because the #25 allows a =
slice
|of the visible spectrum through (from about 590nm).

Depending on which post you're referring to, there is some confusion
here.  The publication in question shows six otherwise identical
landscapes:
        With pan film, no filtration
        With pan film, polarized
        With pan film, #25 plus polarized
        With IR film, #25
        With IR film, #87
        With IR film, #87 plus polarizer

What's missing, obviously, is a comparison of IR film and the #25
filter with and without a polarizer.  Joe evidently was trying to draw
conclusions about that based on the behavior with the #87 filter,
where the difference is admittedly very slight.  I was trying to draw
conclusions based on what I saw through the viewfinder, where the
difference is dramatic.  The truth, as usual lies between the two
extremes.  Allowing for the fact that I was not seeing the IR that
would be determining the majority of the exposure, I expected
appreciable change.  My negatives suggest a little change--perhaps
enough to be worth the bother under some rather limited circumstances,
but these negatives aren't a very good index as it turns out.

Based on my experience, however, it appears that if you were using the
new Ilford film, whose reach into the IR is limited, and you want dark
skies, the use of a polarizer in addition to the red filter could be
very effective.

Bob Long
(boblong@xxxxxxxxxxx)

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Topic No. 2