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Re: [OM] Re: circ. polarizer question

Subject: Re: [OM] Re: circ. polarizer question
From: "Walt Wayman" <hiwayman@xxxxxxxxx>
Date: Mon, 23 Dec 2002 08:02:23 -0500

This is all very interesting, but it?s kind of like asking
someone what time it is and being given instructions for building
a clock.  The question was which way round to put the glass back
in the ring, and my answer was the essence of simplicity AND was
correct: Just look through the damn thing!

I have now, this morning, after only a half cup of coffee,
verified my hypothesis by making like Popeye and squinting
through each of my circular polarizers, three B+W?s, one Tiffen
and one Hoya.  Looking from the back, or camera, side of the
filter and rotating it while looking at reflections on a plastic
slide storage sheet, the polarizing effect was obvious -- the
reflections went away!  Turning the filter around and repeating
the exercise from the front, or subject, side produced an equally
obvious and opposite result.

Simple minds seek simple solutions.

Walt

---------- Original Message ----------------------------------
From: "CyberSimian" <CyberSimian2@xxxxxxxxxxxxxx>
Reply-To: olympus@xxxxxxxxxxxxxxx
Date:  Mon, 23 Dec 2002 09:34:14 -0000

>------Original message------
>>Date: Sun, 22 Dec 2002 18:43:36 -0500
>>From: "Walt Wayman" <hiwayman@xxxxxxxxx>
>>Subject: Re: [OM] circ. polarizer question
>
>>Circular polarizers only work in one direction.
>
>That is true; but they polarise in both directions.
>
>>Looking through it at a shiny surface should give you the
>>answer.
>
>No, it won't.  A circular polariser consists of two "glass"
>elements.  The first element (the one nearest the subject) is
>the polarising element.
>Light from the subject strikes the polarising element and
>emerges polarised.
>The polarised light then strikes the second element, which is a
>randomiser -- it takes the polarised light and randomises the
>polarisation,
>so that the light emerging from the second element is
>unpolarised  This is necessary to prevent autofocus sensors and
spot meters giving incorrect readings.  So:
>
>Polariser fitted correctly:
>  Unpolarised light from subject strikes first element, emerges
>polarised, strikes second element, emerges unpolarised, then
>strikes front element of camera lens.  This gives correct spot
>readings.
>
>Polariser fitted reversed:
>  Unpolarised light from subject strikes second element, emerges
>unpolarised, strikes first element, emerges polarised, then
>strikes front element of camera lens.  This gives incorrect spot
>readings.
>
>Now, the whole point of using a polariser is selectively to
>reduce the amount of light in certain parts of the picture (e.g.
>the sky) by eliminating polarised light originating from that
>part of the picture. So of course you have polarised light
>striking the first element of the polariser.
>But light with the incorrect polarisation is eliminated by the
>first element, so that the light striking the second element is
>still polarised, but there is less of it (e.g. the sky is darker
>than it otherwise would be).
>
>The way to determine which way round a loose polariser should be
>fitted is to use a second circular polariser.  Fit them together
>and then rotate one with respect to the other whilst looking
>through them (you don't need to look at polarised light).  If
>there is no (or little) darkening as one polariser is rotated
>with respect to the other, they are fitted correctly.
>If the image darkens to complete blackness, one polariser is
>fitted the wrong way round relative to the other.
>
>Although a circular polariser consists of two elements, they may
>be cemented together, so that you actually have only one piece
>of "glass".
>
>-- from Cy in the UK
>







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