How do circular polarizers work.

I understand one is supposed to use a circular polarizer with autofocus
cameras. I don't use one yet, as my digicam does not have a convenient
way to mount a filter, and my film cameras are manual focus. However, I
am considering getting a filter mount for the digicam, and realize I
don't understand how to use a circular polarizer.

With a normal (linear) polarizer, I darken sky by rotating the plane of
the filter polarizer so it is close to 90 degrees from the polarization
vector of the sky. However, doesn't the circular polarizer just rotate
the vector of any existing polarization in the scene? How does that
darken anything? If the filter, say, rotates vertically polarized
skylight to horizontal, what does that buy me?
--
Don Stauffer in Minnesota

webpage- http://www.usfamily.net/web/stauffer

On Sat, 24 Jul 2004 08:42:21 -0500, Don Stauffer wrote:

I understand one is supposed to use a circular polarizer with autofocus
cameras. I don't use one yet, as my digicam does not have a convenient
way to mount a filter, and my film cameras are manual focus. However, I
am considering getting a filter mount for the digicam, and realize I
don't understand how to use a circular polarizer.

With a normal (linear) polarizer, I darken sky by rotating the plane of
the filter polarizer so it is close to 90 degrees from the polarization
vector of the sky. However, doesn't the circular polarizer just rotate
the vector of any existing polarization in the scene? How does that
darken anything? If the filter, say, rotates vertically polarized
skylight to horizontal, what does that buy me?

No, it's not like you think. A circular polarizer is simply a linear
polarizer in front of a 1/4-wave retardation plate. The retardation plate
(in effect) removes the polarization of the light that has passed through
the polarizer so that as it gets bounced around any beam splitters - which
are polarized by their nature - they behave just as they would have done had
you not put the filter in the light path, so AF and metering isn't scewed
up. All the things you want a polarizer to do to the image happens as the
light goes through the polarizing element and is unaffected by the
retardation plate, so it's a win-win situation. Sorry if that's as clear as
mud, best I could do. Maybe someone has a web reference that explains it
better.

--
John Bean

A cynic is not merely one who reads bitter lessons from the past, he is one
who is prematurely disappointed in the future (Sidney J. Harris)

On Sat, 24 Jul 2004 08:42:21 -0500, Don Stauffer wrote:

I understand one is supposed to use a circular polarizer with autofocus
cameras. I don't use one yet, as my digicam does not have a convenient
way to mount a filter, and my film cameras are manual focus. However, I
am considering getting a filter mount for the digicam, and realize I
don't understand how to use a circular polarizer.

With a normal (linear) polarizer, I darken sky by rotating the plane of
the filter polarizer so it is close to 90 degrees from the polarization
vector of the sky. However, doesn't the circular polarizer just rotate
the vector of any existing polarization in the scene? How does that
darken anything? If the filter, say, rotates vertically polarized
skylight to horizontal, what does that buy me?

No, it's not like you think. A circular polarizer is simply a linear
polarizer in front of a 1/4-wave retardation plate. The retardation plate
(in effect) removes the polarization of the light that has passed through
the polarizer so that as it gets bounced around any beam splitters - which
are polarized by their nature - they behave just as they would have done had
you not put the filter in the light path, so AF and metering isn't scewed
up. All the things you want a polarizer to do to the image happens as the
light goes through the polarizing element and is unaffected by the
retardation plate, so it's a win-win situation. Sorry if that's as clear as
mud, best I could do. Maybe someone has a web reference that explains it
better.

--
John Bean

A cynic is not merely one who reads bitter lessons from the past, he is one
who is prematurely disappointed in the future (Sidney J. Harris)

Don Stauffer wrote in
:

I understand one is supposed to use a circular polarizer with autofocus
cameras. I don't use one yet, as my digicam does not have a convenient
way to mount a filter, and my film cameras are manual focus. However, I
am considering getting a filter mount for the digicam, and realize I
don't understand how to use a circular polarizer.

With a normal (linear) polarizer, I darken sky by rotating the plane of
the filter polarizer so it is close to 90 degrees from the polarization
vector of the sky. However, doesn't the circular polarizer just rotate
the vector of any existing polarization in the scene?

No. The polarizing screen works exactly the same as a Linear
Polarizer.

The addition is another screen, called a quarter-wave plate, between
the polarizing screen and the camera. What this does is essentially
"rescramble" the polarized light so that it is no longer polarized, which
prevents it from interfering with beam-splitters and internal filters that
don't respond well to polarized light.

While this sounds like it defeats itself, the light has already been
filtered the way you want by the first screen - the bad ol' reflections
(from sky or surface) have been removed, so the light hitting the 1/4-wave
plate has already been pre-selected, as it were.

See http://dpfwiw.com/polarizer.htm for all sorts of fun stuff,
including illustrations.


- Al.

--
To reply, insert dash in address to separate G and I in the domain

Don Stauffer wrote in
:

I understand one is supposed to use a circular polarizer with autofocus
cameras. I don't use one yet, as my digicam does not have a convenient
way to mount a filter, and my film cameras are manual focus. However, I
am considering getting a filter mount for the digicam, and realize I
don't understand how to use a circular polarizer.

With a normal (linear) polarizer, I darken sky by rotating the plane of
the filter polarizer so it is close to 90 degrees from the polarization
vector of the sky. However, doesn't the circular polarizer just rotate
the vector of any existing polarization in the scene?

No. The polarizing screen works exactly the same as a Linear
Polarizer.

The addition is another screen, called a quarter-wave plate, between
the polarizing screen and the camera. What this does is essentially
"rescramble" the polarized light so that it is no longer polarized, which
prevents it from interfering with beam-splitters and internal filters that
don't respond well to polarized light.

While this sounds like it defeats itself, the light has already been
filtered the way you want by the first screen - the bad ol' reflections
(from sky or surface) have been removed, so the light hitting the 1/4-wave
plate has already been pre-selected, as it were.

See http://dpfwiw.com/polarizer.htm for all sorts of fun stuff,
including illustrations.


- Al.

--
To reply, insert dash in address to separate G and I in the domain

In article ,
Don Stauffer wrote:

I understand one is supposed to use a circular polarizer with autofocus
cameras. I don't use one yet, as my digicam does not have a convenient
way to mount a filter, and my film cameras are manual focus. However, I
am considering getting a filter mount for the digicam, and realize I
don't understand how to use a circular polarizer.

With a normal (linear) polarizer, I darken sky by rotating the plane of
the filter polarizer so it is close to 90 degrees from the polarization
vector of the sky. However, doesn't the circular polarizer just rotate
the vector of any existing polarization in the scene? How does that
darken anything? If the filter, say, rotates vertically polarized
skylight to horizontal, what does that buy me?

Circular and linear polarizers work exactly the same way. No difference
as the polarizer in a circular is exactly the same as the one in a
linear pol. The difference is the addition of a quarter wave plate
sandwiched behind the linear pol foil to allow the meter/focus/finder
display to work properly under all lighting conditions.

--
To reply no_ HPMarketing Corp.

In article ,
Don Stauffer wrote:

I understand one is supposed to use a circular polarizer with autofocus
cameras. I don't use one yet, as my digicam does not have a convenient
way to mount a filter, and my film cameras are manual focus. However, I
am considering getting a filter mount for the digicam, and realize I
don't understand how to use a circular polarizer.

With a normal (linear) polarizer, I darken sky by rotating the plane of
the filter polarizer so it is close to 90 degrees from the polarization
vector of the sky. However, doesn't the circular polarizer just rotate
the vector of any existing polarization in the scene? How does that
darken anything? If the filter, say, rotates vertically polarized
skylight to horizontal, what does that buy me?

Circular and linear polarizers work exactly the same way. No difference
as the polarizer in a circular is exactly the same as the one in a
linear pol. The difference is the addition of a quarter wave plate
sandwiched behind the linear pol foil to allow the meter/focus/finder
display to work properly under all lighting conditions.

--
To reply no_ HPMarketing Corp.

John Bean - typed:
On Sat, 24 Jul 2004 08:42:21 -0500, Don Stauffer wrote:

I understand one is supposed to use a circular polarizer with
autofocus cameras. I don't use one yet, as my digicam does not have
a convenient way to mount a filter, and my film cameras are manual
focus. However, I am considering getting a filter mount for the
digicam, and realize I don't understand how to use a circular
polarizer.

With a normal (linear) polarizer, I darken sky by rotating the plane
of the filter polarizer so it is close to 90 degrees from the
polarization vector of the sky. However, doesn't the circular
polarizer just rotate the vector of any existing polarization in the
scene? How does that darken anything? If the filter, say, rotates
vertically polarized skylight to horizontal, what does that buy me?

No, it's not like you think. A circular polarizer is simply a linear
polarizer in front of a 1/4-wave retardation plate. The retardation
plate (in effect) removes the polarization of the light that has
passed through the polarizer so that as it gets bounced around any
beam splitters - which are polarized by their nature - they behave
just as they would have done had you not put the filter in the light
path, so AF and metering isn't scewed up. All the things you want a
polarizer to do to the image happens as the light goes through the
polarizing element and is unaffected by the retardation plate, so
it's a win-win situation. Sorry if that's as clear as mud, best I
could do. Maybe someone has a web reference that explains it better.

http://www.fas.harvard.edu/~scdiroff...arization.html
or the same link:
http://tinyurl.com/6munv
--
The map is not the territory

John Bean - typed:
On Sat, 24 Jul 2004 08:42:21 -0500, Don Stauffer wrote:

I understand one is supposed to use a circular polarizer with
autofocus cameras. I don't use one yet, as my digicam does not have
a convenient way to mount a filter, and my film cameras are manual
focus. However, I am considering getting a filter mount for the
digicam, and realize I don't understand how to use a circular
polarizer.

With a normal (linear) polarizer, I darken sky by rotating the plane
of the filter polarizer so it is close to 90 degrees from the
polarization vector of the sky. However, doesn't the circular
polarizer just rotate the vector of any existing polarization in the
scene? How does that darken anything? If the filter, say, rotates
vertically polarized skylight to horizontal, what does that buy me?

No, it's not like you think. A circular polarizer is simply a linear
polarizer in front of a 1/4-wave retardation plate. The retardation
plate (in effect) removes the polarization of the light that has
passed through the polarizer so that as it gets bounced around any
beam splitters - which are polarized by their nature - they behave
just as they would have done had you not put the filter in the light
path, so AF and metering isn't scewed up. All the things you want a
polarizer to do to the image happens as the light goes through the
polarizing element and is unaffected by the retardation plate, so
it's a win-win situation. Sorry if that's as clear as mud, best I
could do. Maybe someone has a web reference that explains it better.

http://www.fas.harvard.edu/~scdiroff...arization.html
or the same link:
http://tinyurl.com/6munv
--
The map is not the territory

Circular pols are used in teh same way as linear pols. The label tends to
mis-lead people.

--
http://www.chapelhillnoir.com
home of The Camera-ist's Manifesto
The Improved Links Pages are at
http://www.chapelhillnoir.com/links/mlinks00.html
A sample chapter from my novel "Haight-Ashbury" is at
http://www.chapelhillnoir.com/writ/hait/hatitl.html
"Don Stauffer" wrote in message
...
I understand one is supposed to use a circular polarizer with autofocus
cameras. I don't use one yet, as my digicam does not have a convenient
way to mount a filter, and my film cameras are manual focus. However, I
am considering getting a filter mount for the digicam, and realize I
don't understand how to use a circular polarizer.

With a normal (linear) polarizer, I darken sky by rotating the plane of
the filter polarizer so it is close to 90 degrees from the polarization
vector of the sky. However, doesn't the circular polarizer just rotate
the vector of any existing polarization in the scene? How does that
darken anything? If the filter, say, rotates vertically polarized
skylight to horizontal, what does that buy me?
--
Don Stauffer in Minnesota

webpage- http://www.usfamily.net/web/stauffer