Results 1 to 19 of 19

Thread: Circular & Linear Polarizers

  1. #1
    Member
    Join Date
    Jan 2002
    Location
    North East of SG
    Posts
    704

    Default Circular & Linear Polarizers

    What's the diff?

  2. #2
    Senior Member
    Join Date
    Feb 2002
    Location
    Perth Australia
    Posts
    2,548

    Default Re: Circular & Linear Polarizers

    Originally posted by lefei
    What's the diff?
    The difference is how the polarizer deals with light, linear polarizers polarize light in a linear fashion, while a circular polarizer consists of a linear polarizer and a 'depolarizer' that scatters the linearized light in a random manner.

    There are no differences to the result on film between linear and circular polarizers.

    However, Linear polarizers generally intefere with the auto focus sensors on AF cameras and therefore you should always use a circular polarizer with an Auto focus lens. In some cases linear polarizers will also intefere with modern exposure metering systems and lead to exposure errors.
    Last edited by Ian; 25th May 2002 at 12:29 AM.
    The Ang Moh from Hell
    Professional Photography - many are called, few are chosen!

  3. #3

    Default Re: Re: Circular & Linear Polarizers

    Originally posted by Ian


    The difference is how the polarizer deals with light, linear polarizers polarize light in a linear fashion, while a circular polarizer consists of a linear polarizer and a 'depolarizer' that scatters the linearized light in a random manner.

    Actually, a linear polariser consists of only a layer of polarising film. But a circular polariser consists of both a layer of polarising film plus a quarter-wave plate. You have to learn some physics of light at university level to know what is a quarter-wave plate. I had forgotten......

    a circular polariser actually polarise light in a same manner as a linear polariser, just that the behaviour of the polarised light is different. Therefore, circularly polarised light does not interfere with the metering system of an AF camera
    Last edited by mervlam; 25th May 2002 at 02:00 AM.

  4. #4
    Senior Member
    Join Date
    Jan 2002
    Location
    Northwest
    Posts
    5,011

    Default Re: Re: Re: Circular & Linear Polarizers

    Originally posted by mervlam


    Actually, a linear polariser consists of only a layer of polarising film. But a circular polariser consists of both a layer of polarising film plus a quarter-wave plate. You have to learn some physics of light at university level to know what is a quarter-wave plate. I had forgotten......

    a circular polariser actually polarise light in a same manner as a linear polariser, just that the behaviour of the polarised light is different. Therefore, circularly polarised light does not interfere with the metering system of an AF camera
    I believe the quater wave plate rotates half of the linearly polarised light by 90 degrees as well as delay the propagation by a quarter wavelength. The resulting wave has 2 components of E field, orthorgonal to each other and with 90 degrees phase difference. If you plot the E field at any point against time, you actually get a vector turning in a circle with constant amplitude. That's why it is called a circular polariser. Just my guess.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

  5. #5

    Default Re: Re: Re: Re: Circular & Linear Polarizers

    Originally posted by roygoh


    I believe the quater wave plate rotates half of the linearly polarised light by 90 degrees as well as delay the propagation by a quarter wavelength. The resulting wave has 2 components of E field, orthorgonal to each other and with 90 degrees phase difference. If you plot the E field at any point against time, you actually get a vector turning in a circle with constant amplitude. That's why it is called a circular polariser. Just my guess.
    ar... yes u are correctly.... u DO remember your work.

  6. #6
    Senior Member
    Join Date
    Jan 2002
    Location
    Northwest
    Posts
    5,011

    Default Re: Re: Re: Re: Re: Circular & Linear Polarizers

    Originally posted by mervlam


    ar... yes u are correctly.... u DO remember your work.
    Hehe...actually did not take physics in U.

    Remembered something about Maxwell's equations from my electrical engineering classes.

    Recently I was reading Issac Asimov's "The Left Hand of the Electron" and in one of the articles discussing Newton and his contemporaries' experiments to understand light, he mentioned about crystals with molecular structures that rotate the polarity of light. The amount of rotation is proportional to the thickness of the crystal the light passes through.

    That's how I got the idea of what a circular polariser actually is. Haven't verified my idea with any physics guru yet.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

  7. #7

    Default

    actually that's a first year engineering physics course here in NTU....

    going off topic liaoz

  8. #8
    Senior Member
    Join Date
    Jan 2002
    Location
    Northwest
    Posts
    5,011

    Default

    OK. Let's get back on topic.

    To lefei,

    Apart from the technical differences between a linear and circular polariser, the main concern for a user is the compatibility with the camera's optical and metering systems.

    More and more cameras are using some form of light-splitting prisms in the internal optical design to cater for exposure metering, autofocusing, as well as view finder applications. Such metering systems will be messed up if the light entering the camera is linearly polarised.

    That's when a circular polariser is required to maintain the compatibility.

    There is not much difference in the price, I believe, and if a camera is compatible with both types, then the effect will also be the same.

    As such, going for a circular polariser will be a better bet.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

  9. #9
    Senior Member
    Join Date
    Jan 2002
    Location
    Northwest
    Posts
    5,011

    Default

    By the way this topic has been discussed numerous times before. Do a search on "polariser" and you will get a lot of useful information.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

  10. #10
    Senior Member
    Join Date
    Feb 2002
    Location
    Perth Australia
    Posts
    2,548

    Default Re: Re: Re: Circular & Linear Polarizers

    Originally posted by mervlam


    Actually, a linear polariser consists of only a layer of polarising film. But a circular polariser consists of both a layer of polarising film plus a quarter-wave plate. You have to learn some physics of light at university level to know what is a quarter-wave plate. I had forgotten......

    a circular polariser actually polarise light in a same manner as a linear polariser, just that the behaviour of the polarised light is different. Therefore, circularly polarised light does not interfere with the metering system of an AF camera
    This will teach me to give a simple answer with an admittedly poor choice in wording

    However, the net result is that a retarder (the correct name for a 1/4 wave plate) as Roy has pointed out gives a delay in propogation along one of the two output axis.

    Oh it's almost 18 years since I completed my MSc so the old brain is starting to ossify big time.
    The Ang Moh from Hell
    Professional Photography - many are called, few are chosen!

  11. #11
    Senior Member
    Join Date
    Feb 2002
    Location
    Perth Australia
    Posts
    2,548

    Default Re: Re: Re: Re: Re: Re: Circular & Linear Polarizers

    Originally posted by roygoh

    Recently I was reading Issac Asimov's "The Left Hand of the Electron" and in one of the articles discussing Newton and his contemporaries' experiments to understand light, he mentioned about crystals with molecular structures that rotate the polarity of light. The amount of rotation is proportional to the thickness of the crystal the light passes through.

    That's how I got the idea of what a circular polariser actually is. Haven't verified my idea with any physics guru yet.
    The material is Calcite and it's used in the Glan-Thompson polarizer design that is used in instrumentation where a wide optical bandwidth is required amongst other considerations.
    The Ang Moh from Hell
    Professional Photography - many are called, few are chosen!

  12. #12
    Member
    Join Date
    Jan 2002
    Location
    North East of SG
    Posts
    704

    Default

    Originally posted by roygoh
    By the way this topic has been discussed numerous times before. Do a search on "polariser" and you will get a lot of useful information.
    not much info on the search...anyway, i've gotten a circular polarizer

    thanks to everyone that answered to this thread

  13. #13

    Default Re: Re: Re: Re: Circular & Linear Polarizers



    This will teach me to give a simple answer with an admittedly poor choice in wording



    Didn't know that my English is that bad..... Maybe that happens after 30 months in the SAF



    However, the net result is that a retarder (the correct name for a 1/4 wave plate)


    But 1/4 wave plates are stated as a mica quarter-wave plates on most American physics textbooks.

  14. #14
    Senior Member
    Join Date
    Feb 2002
    Location
    Perth Australia
    Posts
    2,548

    Default Re: Re: Re: Re: Re: Circular & Linear Polarizers

    Originally posted by mervlam
    But 1/4 wave plates are stated as a mica quarter-wave plates on most American physics textbooks.
    Since when were the Yanks known for using correct terminology
    The Ang Moh from Hell
    Professional Photography - many are called, few are chosen!

  15. #15

    Default Re: Re: Re: Re: Re: Re: Circular & Linear Polarizers

    Originally posted by Ian


    Since when were the Yanks known for using correct terminology
    Aussies and the Brits can't be correct all the time either.

  16. #16

    Default

    If i have 2 lenses with different filter size eg 55mm and 62mm. what is the converter called? or there is no such thing? meaning i have to purchase 55mm and 62mm filters...

  17. #17

    Default

    You can purchase a 55 to 62mm step-up ring. You can screw the ring on a 55mm thread lens and screw-in a 62mm filter.

  18. #18
    Senior Member
    Join Date
    Jan 2002
    Location
    Singapore, Singapore, Singapor
    Posts
    6,405

    Default

    Originally posted by stl
    If i have 2 lenses with different filter size eg 55mm and 62mm. what is the converter called? or there is no such thing? meaning i have to purchase 55mm and 62mm filters...
    You SHOULD buy a 55mm and 6mm filters, helps to boost the economy.

    Otherwise, you can get a stepping ring of the correct size. E.g. for your example, you would get a 62mm polarizer filter, and get a 55 (lens)->62mm (filter) stepping ring.

    Regards
    CK

  19. #19
    Senior Member
    Join Date
    Jan 2002
    Location
    Clementi
    Posts
    6,580

    Default

    Originally posted by ckiang


    You SHOULD buy a 55mm and 6mm filters, helps to boost the economy.

    Otherwise, you can get a stepping ring of the correct size. E.g. for your example, you would get a 62mm polarizer filter, and get a 55 (lens)->62mm (filter) stepping ring.

    Regards
    CK
    Am I correct to assume that generally, u put a larger filter over a smaller lens? This way, there is no danger of vignetting? How much difference in diameter can be tolerated by the camera in general? I have a 49mm and a 72mm lens, I guess that is way too far? And the next lens I am going to get is 77mm in diameter. Would it be okay for 72mm to step up to 77mm? BTW, the 77mm is an ultra wide.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •