Question on Circular polarizer and rotating lens element

Discussion in 'Digital Photography' started by PGPS, Sep 21, 2007.

  1. PGPS

    PGPS Guest

    Hi,
    If I am using a circular polarizer on a digital SLR, I heard that a
    rotating lens element will rotate the polarizer and it will basically
    make it useless. As I understand optics, a circular polarizer
    basically changes the phase between the electrical and magnetic
    field.

    1. Am I right in saying that a cirucular polarizer is basically a
    linerar polarizer + a quarter wave (QW) plate.
    2. So, its the rotation of the linear polarizer element which causes
    the problem, since it can either polarize or not-polarize (according
    to the orientation).

    Any suggestions, links, etc.?

    Thanks.
     
    PGPS, Sep 21, 2007
    #1
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  2. PGPS wrote:
    > Hi,
    > If I am using a circular polarizer on a digital SLR, I heard that a
    > rotating lens element will rotate the polarizer and it will basically
    > make it useless. As I understand optics, a circular polarizer
    > basically changes the phase between the electrical and magnetic
    > field.
    >
    > 1. Am I right in saying that a cirucular polarizer is basically a
    > linerar polarizer + a quarter wave (QW) plate.
    > 2. So, its the rotation of the linear polarizer element which causes
    > the problem, since it can either polarize or not-polarize (according
    > to the orientation).
    >
    > Any suggestions, links, etc.?
    >



    Basically what you say is true. Note that the actual polarizer element
    faces to the subject and the quarter wave place is toward the camera.

    If the lens front rotates, you must either rotate the polarizer
    to the correct position after it stops, or, if that is impossible,
    simply use your third hand to hold the polarizer in front of the
    lens, not attached to it. This works if you have even a modestly
    steady hand.

    Doug McDonald
     
    Doug McDonald, Sep 21, 2007
    #2
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  3. PGPS

    Scott W Guest

    PGPS wrote:
    > Hi,
    > If I am using a circular polarizer on a digital SLR, I heard that a
    > rotating lens element will rotate the polarizer and it will basically
    > make it useless. As I understand optics, a circular polarizer
    > basically changes the phase between the electrical and magnetic
    > field.
    >
    > 1. Am I right in saying that a cirucular polarizer is basically a
    > linerar polarizer + a quarter wave (QW) plate.
    > 2. So, its the rotation of the linear polarizer element which causes
    > the problem, since it can either polarize or not-polarize (according
    > to the orientation).
    >
    > Any suggestions, links, etc.?
    >
    > Thanks.
    >


    Well to start it does not change the phase between the electrical and
    magnetic fields, it changes the phase between two orientations of
    polarization.

    The quarter wave plate is on the camera side of the filter as far as hot
    it filters light coming in it acts just light a linear filter, but the
    light that exits the filter is circular. The reason not just letting
    the linear light continue through the camera is many cameras get their
    light meters messed up when using linear polarized light.

    An easy way to look at this is look through the filter as the camera
    would and rotate it, you will see the normal effects, now look through
    it backwards and you should not see any effects of polarization.

    Scott
     
    Scott W, Sep 21, 2007
    #3
  4. In article <>,
    PGPS <> wrote:

    > Hi,
    > If I am using a circular polarizer on a digital SLR, I heard that a
    > rotating lens element will rotate the polarizer and it will basically
    > make it useless. As I understand optics, a circular polarizer
    > basically changes the phase between the electrical and magnetic
    > field.
    >
    > 1. Am I right in saying that a cirucular polarizer is basically a
    > linerar polarizer + a quarter wave (QW) plate.
    > 2. So, its the rotation of the linear polarizer element which causes
    > the problem, since it can either polarize or not-polarize (according
    > to the orientation).
    >
    > Any suggestions, links, etc.?
    >
    > Thanks.


    Enter Circular polarizer into Google. Lots of information available.

    --
    Es ist nichts schrecklicher als eine tätige Unwissenheit.

    Johann Wolfgang von Goethe
     
    Ockham's Razor, Sep 21, 2007
    #4
  5. PGPS

    Marty Fremen Guest

    PGPS <> wrote:
    >
    > If I am using a circular polarizer on a digital SLR, I heard that a
    > rotating lens element will rotate the polarizer and it will basically
    > make it useless.


    Probably precisely because of that problem, not many lenses rotate their
    front element / filter mount when focussing or zooming. They used to maybe
    30+ years ago but I've not had any cameras or lenses in the last 25 years
    which suffered a rotating front element. Even my point and shoot digital
    doesn't (not that it has a filter thread anyhow). I don't have a DSLR
    though - maybe there's been some regression in lens design since DSLRs came
    in?

    If you do have such a lens, it's not a huge problem, since the filter
    will freely rotates in its mount whilst attaced to the camera. Either take
    hold of it in one hand whilst focussing/zooming to stop it rotating, or
    else correct its orientation afterwards.
     
    Marty Fremen, Sep 21, 2007
    #5
  6. PGPS

    Joe Miller Guest

    Re: Question on Circular polarizer and rotating lens element- incorrect statement

    > An easy way to look at this is look through the filter as the camera
    > would and rotate it, you will see the normal effects, now look through
    > it backwards and you should not see any effects of polarization.


    It is likely you will still see an affect looking through the filter
    backwards, but it depends on the details of the scene. Suppose the angle
    of the polarized light from the scene lines up with the fast axis of the
    quarter-wave plate. Then that plate won't do anything to the poloarized
    light, but the linear polaroid behind it will diminish its intensity;
    it's at a different angle. So as you rotate the filter you will see some
    effect as the angle of the plate and polaroid interact with the scene
    polarization. This is a minor point, but I didn't want someone to think
    their filter was faulty if they tried this and saw an effect on the
    light. Take two circular polarizers or one circular and one linear and
    change their order or flip them and you can get some interesting,
    different effects as you rotate them.

    Joe
     
    Joe Miller, Sep 21, 2007
    #6
  7. PGPS

    Joe Miller Guest

    In article <>,
    PGPS <> wrote:

    > Hi,
    > If I am using a circular polarizer on a digital SLR, I heard that a
    > rotating lens element will rotate the polarizer and it will basically
    > make it useless. As I understand optics, a circular polarizer
    > basically changes the phase between the electrical and magnetic
    > field.
    >
    > 1. Am I right in saying that a cirucular polarizer is basically a
    > linerar polarizer + a quarter wave (QW) plate.
    > 2. So, its the rotation of the linear polarizer element which causes
    > the problem, since it can either polarize or not-polarize (according
    > to the orientation).
    >
    > Any suggestions, links, etc.?
    >
    > Thanks.


    Points (1) and (2) are correct. The light first goes through the linear
    polaroid. It does all the work on the light that you wish to have
    happen. The quarter-wave plate, which converts the linear to circular
    polarization is there for the benefit of downstream camera elements. So,
    if your lens rotates the filter, you must re-set the filter before
    shooting to the angle you want. There's no way around that.

    Joe
     
    Joe Miller, Sep 21, 2007
    #7
  8. PGPS

    Pete Guest

    On 21 Sep 2007 19:19:09 GMT, Marty Fremen wrote:

    > Probably precisely because of that problem, not many lenses rotate their
    > front element / filter mount when focussing or zooming. They used to maybe
    > 30+ years ago but I've not had any cameras or lenses in the last 25 years
    > which suffered a rotating front element. Even my point and shoot digital
    > doesn't (not that it has a filter thread anyhow). I don't have a DSLR
    > though - maybe there's been some regression in lens design since DSLRs came
    > in?


    There has indeed been such a regression. The Nikon D40 DLSR kit lens has a
    rotating front element. PITA.

    > If you do have such a lens, it's not a huge problem, since the filter
    > will freely rotates in its mount whilst attaced to the camera. Either take
    > hold of it in one hand whilst focussing/zooming to stop it rotating, or
    > else correct its orientation afterwards.


    Because of the slop in the focusing mechanism, you really need to...

    1. focus
    2. adjust polarizer
    3. remove fingers from polarizer
    4. focus again.

    This is a significant problem if you need to take fast shots.
     
    Pete, Sep 26, 2007
    #8
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