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    Re: Sextants with Polarizing filters
    From: Bill B
    Date: 2006 Jan 26, 20:25 -0500

    Bill wrote:
    |
    | As per your request, did a quick series of tests using a tripod mounted
    | Minolta digital flash meter, pointed at a flat white interior wall
    | illuminated by an incandescent bulb.
    |
    | Baseline reading, no filter    f5.6  .9
    | With Vivitar 77mm filter       f4.0  .4
    | With Hoya 67mm filter          f4.0  .6
    | With Nikon 52mm filter         f4.0  .5
    | 77mm and 67mm combined         f4.0  .0
    
    Frank asked
    
    "That last observation is  bit of a surprise to me. And what does the second
    figure represent, that follows the f-number?"
    
    It is a digital meter, so reads out f-stops to the tenth.  For clarity the
    .X numbers on the display are in smaller-size fonts/digits than 5.6, for
    example, to avoid confusion. Stops like 2.0 and 4.0, appear with the .0 for
    like reason I suspect.  5.6.9 is just confusing.  Imagine f-1 + 4 tenths
    reading out as 1.4, or f2 + 8 tenths reading out as f2.8. Close but....
    
    George added:
    
    "I would have expected the two polaroids combined (and aligned) to let
    through a bit less light than a single polaroid does, simply because of
    surface reflections and imperfections in the transmission. But I thank Bill
    for confirming the point that I was trying to make, in numerical terms."
    
    My pleasure.  You have been very generous with your time in mentoring me and
    others. BTW, 2 filters stacked at 90d reduced transmission by approx. 10.5
    stops.
    
    Take it as one for the win column if you wish.  Commons sense would suggest
    if 2 identical filters were face to face and in register, you would have
    beaten the point spread as well.   But to date practical experience and
    models based on other filters have failed the common sense test relative to
    polarizing filters for me, so ignore the man behind the curtain ;-)
    
    Thanks to all for the many links with varying levels of explanations.  The
    grade-school model of the second fence realigning the spring motion was
    about my speed ;-)
    
    Off topic, I have always wondered about the order of the f-stops.  The
    relationship is clear enough: relative increase in the radius of the
    aperture to double transmitted light.  It has always struck me as strange
    given aperture dimensions that 1, 1.4, 2 etc are the largest openings, while
    45, 64, and 90 are much smaller.  An inverse relationship.
    
    Any history buffs out there than can explain that?
    
    Bill
    
    
    

       
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