Re: polarizing filters with b&w infrared

[Scott Donkin <photos@xxxxxxxxxx>]

At 05:08 PM 30/10/96 +0000, you wrote:

>A few days ago there was a question regarding the use of polarizing filters
>with b&w infrared film. The person noted that in Kodak's Applied Infrared
>Photography book ( which I also own ) there were two examples, one was a shot
>taken with an 87 filter only and one with an 87 and polarizer. The writer
>noticed that the shot with both filters had more contrast and assumed a
>polarizer should be used in addition to a no.25 or 29 red filter as well. My
>response is that if you look at the two photos , you would be hard pressed to
>notice any difference at all in contrast , very minimal at best. Also take
>into account that 2-2 1/2 stops of light would be cut out with use of  a
> polarizer in addition to the light absorbed by a red or infrared filter. Not
>a good or practical combination of filters for these reasons and besides
>polarizing filters can only polarize visible light, not infrared light.

As I mentioned in an earlier post...

You'll find that the use of a polarizing filter for IR wavelengths alone
will have quite a reduced effect.  Ie, polarizing filters do not polarize
long(er) wavelengths (IR end) as efficiently as shorter wavelengths (UV end).

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).

As far as how much light loss you get with a polariser, with or without
other filters, when using IR film, you'd have to experiment.  There is no
guarrantee you would lose the normal 2 stops with the polariser when using
IR - especially considering pola filters have less effect as you move into
the longer wavelengths.


Cya...

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Topic No. 3