Re: Neutral Density Filters

[w.j.markerink@xxxxx (Willem-Jan Markerink)]

In article <6nb3f2$3rp@xxxxxxxxxxxxxxxxxx>,
   bcasselb@xxxxxxxxxxx (Bill D. Casselberry) wrote:
>
>  Scot posts, along w/ a useful list ...
>
>Each ND filter will change your shutter speed by a number of stops. You 
use
>the value of the filter to figure out how many stops.
>
>	while on the ND subject, I have another question about
>	them. I was curious if anyone knew their infrared trans-
>	mission level. That is, do they reduce just the more normal
>	visible spectrum, but pass the higher (lower ??) IR wave-
>	lenghths? Was wondering if using the one I have ( a Canon
>	8x - 3 stops?) would have an effect of biasing the incoming
>	light towards the infrared or if it would reduce both evenly.
>
>				thanks for any insight on this ...
>
>						   Bill

Both ND and polarizers are notoriously bad in the IR-spectrum....they pass 
IR almost unaffected. A crossed set of polarizers makes a decent IR filter, 
just like a strong ND. 
You can try this with a ND grad and a night vision scope, or a video-cam 
after the light is filtered through a true IR filter.

Note that *some* very strong ND's might not consist of colored dyes, but 
metal (silver?) particles. I am not sure on what microscopic level we are 
talking here (after all, the image shouldn't suffer optically), but I once 
came accross this description. Being metal, they might extend their effect 
in the IR spectrum.
Actually, I recently received a custom made Cokin-P size ND 3.0 (12 stops), 
and I could do some testing with both IR film and a night vision scope.
(a very rare Rollei RF100 unit, for sale at US$3500, in case anyone wants 
to fund my other investments and projects....8-))
Stay tuned!




- --
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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