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#1
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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 |
#2
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How do circular polarizers work.
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) |
#3
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How do circular polarizers work.
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) |
#4
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How do circular polarizers work.
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 |
#5
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How do circular polarizers work.
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 |
#6
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How do circular polarizers work.
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. |
#7
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How do circular polarizers work.
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. |
#8
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How do circular polarizers work.
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 |
#9
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How do circular polarizers work.
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 |
#10
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How do circular polarizers work.
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 |
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