From:             Willem-Jan Markerink 
To:               eos@avocado.pc.helsinki.fi
Date sent:        Thu, 17 Apr 1997 12:56:03 +0000
Subject:          Re: Polarizer vs circular polarizer
Send reply to:    eos@avocado.pc.Helsinki.FI

On 16 Feb 97 at 20:33, Rafael Lemos Bezerra wrote:

> What is the difference between the Polarizer and the circular polarizer ?

A circular has an additional quarter-wave plate or scrambler behind 
the (still linear) polarizing foil. Although not scientifically 
correct, it more or less restores the natural 50/50 vertical/horizontal 
balance of polarization, without affecting the initial pictorial result.

Only by restoring this natural balance it will allow the light 
metering and AF sensors to work properly, as they use polarizing beam 
splitters. With a linear filter, you would risk a cross-polarizing 
effect, ie a black-out. Bad for both light metering and AF.

In spite of what most people will tell you: the main reason to buy a 
circular polarizer is *not* the AF sensor, but the light metering 
system. You can *see* when AF goes haywire (it won't shift focus, it 
just has more difficulty to lock on), but you can only guess what 
happens with your light meter!

Actually, the first circulars were required long before AF existed, 
and are still required for non-AF cameras today (Rollei 600x series 
is a nice example).

PS: please adjust the time in your PC, it reads February instead of
April (your message ends up on the bottom of my mailbox). 
Better stop playing with time-restricted demo software....;-))

-- 
Bye,

       _/      _/       _/_/_/_/_/       _/_/_/_/_/
     _/  _/  _/               _/       _/  _/  _/
     _/  _/ illem    _/     _/ an    _/  _/  _/ arkerink
                     _/_/_/  



      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


[w.j.markerink@a1.nl]
[note: 'a-one' & 'en-el'!]

Date: Mon, 07 Jul 1997 09:52:53 +0200
From: Nicolas EERTMANS 
Subject: RE:Circular Polarizers:Theory

AF cameras have mirrors that act as beam-splitters: most of the light is
reflected to the viewfinder for metering and viewing, while the rest
(typically 25%) is transmitted and then reflected by a secondary mirror
to the AF sensor, which is in the camera body.

The problem is that the ratio of reflected/transmitted light, fixed for
non-polarized light, varies with the polarization plane orientation of
polarized light, relatively to the beam-splitter, typically between
62%/38% to 88%/12%.

The amount reaching the viewfinder (i.e. the metering cells) varies, but
not that much: +/- 1/4 stop in our example.

But the amount of light reaching the AF sensor varies much more: about
+/- 1 stop in our example.

So, IF your front element rotates during focusing, your AF might be very
much disturbed by the light variation. But if you hold it in position
during focusing (which is what you want to do anyway), you shouldn't
have much problems, as long as there is enough light.

Circular polarizers fix these problems by adding a second layer to a
traditional polarizer. The purpose if this second layer is to depolarize
the light (actually, it polrize it "circularly", hence the name), after
the filtering stage (remember, the aim of the polarizer is not to
polarize the light but to filter it, so, once it is filtered, you can
depolarize it if you want). This fixes the problem at the beam-splitter
level.

For me, its main advantage is to avoid exposure problems, especially if
you have the spot metering cell in the body, as with the F4(*), but
correct AF operation with front-rotating lenses is anecdotical, as you
don't want your filter to turn with focusing...

Hope this helps,

Nicolas Eertmans.

(*) the F601, and probably the F801s (F90 and F70?), do have all their
metering cells, spot included, in the viewfinder.

Date sent:        Tue, 08 Jul 1997 13:58:57 +0200
From:             Nicolas EERTMANS 
Organization:     European Organisation for the Safety of Air Navigation, EUROCONTROL
To:               w.j.markerink@a1.nl
Subject:          Re: circular vs linear explanation

Willem-Jan Markerink wrote:
> 
> Hi Nicolas,
> 
> Would you mind if I added your very nice explanation to my homepage,
> in a chapter explaining circular vs linear? With full credentials of
> course! See:
> http://www.a1.nl/phomepag/markerink/mainpage.htm

No problem at all. Just some precisions: numbers are taken from an
example in the Focal Press Encyclopedia of Photography. They might vary
following the camera model (The FNAC (a french photo-video-cd-book
store) tests give a polarized light inflence of 0.4 stop for the F601,
which is much more than what I personally estimated).
Also, I'm not fully sure the polarized light does not influence the
phase-detection principle used by the AF system.

Nicolas

FROM: w.j.markerink@a1.nl (Willem-Jan Markerink)
SUBJECT: Re: Identifing linear vs circular polarizer.
DATE: Tue, 21 Apr 98 14:06:23 GMT
ORGANIZATION: A1 de Internet Provider uit Twente B.V.
NEWSGROUPS: rec.photo.equipment.misc

In article ,
   Doug Stemke  wrote:
>I was told you can identify a linear vs circular polarizer in that linear
>polarizers only turn part way and circular rotate all the way around the
>filter.  Is this true?  

No, both rotate a full 360 degree.
There are two ways to distinguish them:

- Look at (non-metallic) reflective surface through the polarizer and flip 
it around. A circular won't kill reflections in reversed position, a linear 
will still work.

- Look through the polarizer into the mirror, and flip it around. A linear 
won't change density/transmission, a circular will black out in one of the 
positions (forgot which one, normal or reversed....think it is the normal 
position).

 


--
Bye,

Willem-Jan Markerink


      The desire to understand
is sometimes far less intelligent than
     the inability to understand



[note: 'a-one' & 'en-el'!]






FROM: Andy Resnick 
SUBJECT: Re: Circularly Polarized Light...
DATE: Tue, 12 Oct 1999 09:07:42 -0400
ORGANIZATION: NASA Glenn Research Center
NEWSGROUPS: sci.physics,sci.optics

"David H. Gotz" wrote:

> Can someone out there point me to an online site that can explain what
> the basic idea behind circular polarization of light is?  I can't seem
> to find a site that talks about it.  What are the fundamental
> differences between it and linearly polarized light?
>
> Thanks,
> Dave
>
> PS-Also, if this is the wrong newsgroup for this question, please let me
> know.

David,

Both linearly polarized and circularly polarized light form basis states to
the vector wave equation for electromagnetic radiation.  Any polarization
state can be described in terms of linear combinations of either horizontal
and vertical polarization or left- and right-handed circular polarization.
When solving the equation, textbooks usually present the linear polarization
states because they are easy to write down: the electric field oscillates in
the 'x' or 'y' axis, the magnetic field is perpendicular to that, and away
you go.  Then, they show that a second set of solutions exist- the circular
polarization states, where left or right-handed circular polarization states
are created by having the two linear polarization states be out of phase by
90 degrees. In this case, the electric field vector moved in a circle in the
x-y plane, either clockwise or counter-clockwise. (I forget which is left-
or right-handed)  In any case, it turns out that circular polarization is
actually more fundamental than linear polarization, as individulal photons
are circularly polarized: they carry angular momentum.

Andy Resnick






FROM: ajw29585@aol.com (Ajw29585)
SUBJECT: Re: Circularly Polarized Light...
DATE: 12 Oct 1999 15:27:28 GMT
ORGANIZATION: AOL http://www.aol.com
NEWSGROUPS: sci.optics

For what it's worth:

A real life application is a glare shild for your monitor. It's made by using a
linear polarizer, then a circular polarizing film. The  light from the room is
first linearly polarized, then the circular polarizer rotates  its plane of
polarizarion 45 degrees. It's reflected from the monitor screen, back through
the circular polarizer, where it undergoes another 45 degree rotation, then to
the linear polarizer. Now, however, it's shifted some 90 degrees, and absorbed.
Light emitting from the monitor screen, however, randomly polarized, is shifted
45 degrees, still randomly polarized, and then half of it is absorbed by the
linear polarizer. The balance is transmitted to the room.   

If you have any question, remark, comment, want to share some philosophy or just want to express your opinion about these pages, feel free to send email to: w.j.markerink @ a1.nl