Re: using neutral density filters with IR film.

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On  7 Nov 97 at 23:50, Rolland Rainbow Elliott wrote:

> I thought I read on this list a post (I think by WJ) that stated that neutral
> density filters were transparent to infrared radiation.

Yup, and so are polaroid filters, black plastic garbage can bags and 
non-exposed + developed sheets of E-6 slide film....:-))

> 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?

I once investigated the possibility of having IR-proof ND gradual 
filters made, but the final price soon brought me back to earth....8-))

Related issues:
- - you can create a decent 'IR-filter' by using a non-exposed and 
developed piece of E-6 slide film....almost the same transmission as ND filters.
- - you can create a decent IR filter by cross-polarizing two 
polarizers....
- - and the same goes for ND's....

Note that real IR-filters have a much steeper curve than a ND
filter....you need a pretty dark ND filter before you get a #87C
equivalent....about 4.0 or so (16 stops) is the maximum for resin
squares....and those are not easy to find....I believe only B+W makes
much darker glass ND's for special applications.



- -- 
Bye,

Willem-Jan Markerink


      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]
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