P3D Re: Round or Circular

[Rob <lilindn@xxxxxxxxxxx>]

(Bob Maxey wrote):
> >>How about using circular polarizing filters, would that make it
> easier to do?  I see they're available for cameras.
> 
> What's the difference, and forgive me, I am not sure I understand what you
> mean. Are you referring to a circular polarizer as in circular shape or a
> circular polarizer as in a linear polarizer with the combined retarder to
> make it a circular polarizer - a specific kind of polarizer filter.
..
..
    There is a lot on confusion today about the two kinds of polarizers
now on the market - Linear polarizers (before about 1980 merely called
polarizers) and Circular polarizers.  Both usually come in round frames.
    Linear polarizers are usually less expensive (if you can find them),
whereas circular polarizers are considered more suitable for modern SLR
(2D) cameras with such features as TTL metering and autofocus.
    Both have the same main desired goal - they only stop the
transmission of light in an undesired polarization when turned 90 (or
270) degrees in relation to the undesired light.  This can eliminate
reflections off of glass, greatly reduce glare off of non-reflective
surfaces and deepen the blue of the sky, and slightly penetrate haze in
the visible spectrum.
    The difference is that with a linear polarizer the transmitted light
goes to the camera still polarized.  Many through-the-lens devices are
sensitive to polarization and can produce false exposure readings or 
fail to autofocus under highly polarized light, whereas with a circular
polarizer, the light passing to the camera is somewhat depolarized, and
will not usually interfere with the AF or TTL systems. 
    The term "circular polarizer" is somewhat confusing (IMHO a
downright misnomer).  When one thinks of "circular polarization", one is
inclined to think of a wave with horizontal and vertical components 90
(or 270) deg out of phase with each other, resulting in a "corkscrew"
shape wave (this is what the term means with respect to radio
broadcastng, for example).  One would expect a circular polarizer for
your camera to allow only a circular polarization of one type (e.g.
right-handed) through while absorbing the other (e.g. left-handed). 
These filters to nothing of the sort.  A circular-polarized filter is
actually a linear polarizer with a quarterplate (a depolarizing layer)
behind it.  Perhaps a more correct term would be "polarizer-depolarizer
filter".  Suppose you have a circular pol on your SLR and are taking a
picture with a large area of clear blue sky in the background.  You turn
the filter so that the polarizer layer at front rejects the polarized
light scattered from the air, the quarterplate layer behind it
depolarizes this light so that your camera's electric-eye isn't fooled,
but the circular filter is no less effective than the linear - because
the light is already effectively polarized by the front layer, making it
bluer, and the polarization of the light reaching the film dosen't
matter.
     If you have a circular polarizer and a linear polarizer, try this
experiment: First, hold the linear between you and a light source.  Now,
place the circular between your eyes and the linear filter and turn
either filter.  You will observe that the light can be almost completely
blocked out.  This is because the light is being polarized in one
direction by the linear filter, and can't pass through the polarizing
layer at the front of the circular polarizer - there's no light left the
the quarterplate layer to depolarize.  Now, hold the circular between
you and the light, and place the linear between you and the circular. 
You will see that the light can only partially be blocked by turning the
filter.  This is because the circular filter is polarizing the light,
but then depolarizing it, and the linear filter can't completely block
the light because it is not strongly polarized.
     For stereo cameras, choose a polarizer for price.  I've never seen
a stereo camera with TTL metering, nor autofocus, so it doesn't make any
difference which type you use.
     Rob
     "Everything I have is Y1.96K compliant"
BTW - if anyone should start producing polarizers for Realist 3.5,
please let me know.


------------------------------