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    Re: Simplified Bris Sextant (calibration)
    From: Bill B
    Date: 2012 May 15, 17:35 -0400

    On 5/15/2012 3:21 PM, Greg Rudzinski wrote:
    > You should see a comfortable blue disk with the polarizers filtering the
    > maximum. You can't just stack two polarizers. It is necessary to take
    > one of the polarizers out of the frame and reverse it over then install
    > back into the frame. Test by flipping one of the polarizers 180*
    > relative to the other while off the lens.
    
    Greg
    
    Thanks for this and the following explanation.  I was doing some reading
    on polarizing filters when your message(s) arrived. Some of it above my
    pay grade ;-)
    
    http://www.lenstip.com/115.2-article-Polarizing_filters_test_About_light_and_polarization.html
    
    It looks like it may well be possible to stack circular polarizing
    filters for light reduction as the subject side is a linear and that is
    converted to circular by the next film so it does not foul up the
    prism/two-way mirror ability to calculate exposure and distance (DSLR).
    
    Are your filters of the newer circular or the traditional linear variety?
    
    If traditional linear, I am not clear why one filter has to be flipped.
      Isn't just a film sandwiched between two pieces of glass?
    
    An interesting bit of trivia: Bees use polarization for orientation and
    navigation.  Must have learned that from the Vikings?
    
    Bill B.
    
    
    
    

       
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