Re: Double Circular Polarizers for UV photography? Mounting an

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

On 12 May 00 at 13:04, Rolland Elliott wrote:

> Hi,
> 
> I was wondering if anyone has every tried using two polarizer filters as an 
> equivalent to an 18A filter (this is a UV filter that blocks all visible 
> light, but allows UV and IR light to pass through)

Just be sure to use only foil, no additional ordinary (UV blocking)
glass (as most mounted pol's have). 
Would also require a bit of testing different brands, since pol foil 
is available in quite range of spectral plots, in both single and 
double/cross applications.

> I was also wondering if anyone has ever custom cut an 18A filter (equivalent 
> to a B+W 403)to approximately 24x36mm and mounted it in side a camera behind 
> the mirror but infront of the shutter assembly?

The main problem is that any UV-pass filter will be have to be made
from glass....thin-foil filters do not exist. This makes both a
BTFR-filter almost impossible (definately for 35mm cameras) because
of thickness of glass, but any other custom-sizing in front of the 
shutter also quite difficult in terms of cutting, placing & mounting. 

> I'm trying to get into some UV action flash photography, but the most 
> difficult problem is that one cannot compose or focus through a visibly 
> opaque 18 A filter! By using two circular polarizers, one could simple turn 
> the polarizer right before exposure to block visible light.
> 
> As far as the other solution of mounting a filter between the mirror and 
> shutter, the custom cut filters are expensive. See below for pricing from 
> Rolyn Optics for 25mm x 37mm x 1mm filters
> 1pc    #65.1015    cut to 25mm x 37mm $130.25 ea
> 1pc    #65.1005    cut to 25mm x 37mm $157.50 ea
> 
> Both of the above filters are very close to an 18A filter.

But a 75x75mm Kodak filter for US$200 sounds a much better 
deal....you can cut almost 6 pieces out of that.
 
> Or I was thinking I could just buy a regular round filter and cut it down 
> myself. B+W's filters are only about 2.2 mm thick and I should be able to 
> squeeze one in a mirror box after cutting it down.  However the only kind of 
> glass I've ever cut is window pane glass and filter glass is very different. 
> Do you think using a glass etcher would work on cutting up a round filter to 
> a rectangular shape? I have a Dremel rotary tool that might be useful.  
> Cutting it down myself, would cost me about 1/2 as much as ordering a custom 
> filter (unless of course I broke the filter in the process) then the money 
> would be wasted.

I guess you better post this question to the Usenet newsgroup sci.optics.... 

--                 
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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