Re: using neutral density filters with IR film.

["Sandor Mathe" <sandor.mathe@xxxxxxxx>]

> From:          Rolland Rainbow Elliott <X93ELLIOTT1@xxxxxxxxx>

> In Theresa Airey's Book "Creative Printmaking" she states:
> "Neutral Density filters don't change the tones in an [IR] negative; they
> merely reduce the light entering the lens so that a longer shutter speed is
> required to record the subject.  Of course, the use of an ND filter will also
> permit you to use a large aperature, such as f/2.8 or f/2 for a controleld
> focus background."
> 
> I thought I read on this list a post (I think by WJ) that stated that neutral
> density filters were transparent to infrared radiation.
> I've always assumed this to be true, especially after recording a picture of
> my friend wearing dark sunglasses with a camcorder fitted with an 87C IR
> filter.  My friends sunglasses looked like regular eyeglasses with 
> clear lenses.  
> If this neutral density filters are trans parent to IR light then
> they would not effect the IR light passing through them, but would impact the
> visible light through them.  This suggests that neutral density filters do in
> fact change the tones in an IR negative because they would reduce the amount
> of visible red light passing through without effecting the infrared light
> passing through the filter.  
> 
> This is just all speculation though.  Has anyone used neutral density filters
> with IR films.  And if so what were the results?

Sorry, never used a neutral density with IR. I think you need to get
the absorption/transmission spectrum of the particluar filter (or
test it yourself).  Neutral density filters are usually designed for
visible light only.   I suspect that depending on the materials
selected the IR absorption could be anything from nothing to lots.

If the filter designer didn't care about anything outside the
visible range then the materials they selected would set the
absorbtion by happenstance.

Sandor Mathe  -- sandor.mathe@xxxxxxxx
(905) 670-1225 x333  --  FAX (905) 670-1344
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