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Thread: Two CPLs = Zoomable ND?

  1. #21
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    Default Re: Two CPLs = Zoomable ND?

    Quote Originally Posted by azul123 View Post
    Thanks for your input Roy, saw Sammy888's and your postings on CPL/LP filters and they are indeed very technical and detailed.

    ../azul123
    I remember that discussion! After writing all that the first response I got was that it made it worse for someone who has not got the patience to read. Well...

    I am glad you found that writeup helpful.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

  2. #22
    vince123123
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    Default Re: Two CPLs = Zoomable ND?

    I found it helpful too - I persoally dont have a problem with wordiness

    What I was confused though, is it true that addiing a linear polarizer in front of a circular polarizer will work? I always thought that LPs wont' work with AF, but from what you appear to say, the CP will "restore" the LP back to a compatible format with AF.

    Quote Originally Posted by roygoh View Post
    I remember that discussion! After writing all that the first response I got was that it made it worse for someone who has not got the patience to read. Well...

    I am glad you found that writeup helpful.

  3. #23
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    Default Re: Two CPLs = Zoomable ND?

    Quote Originally Posted by vince123123 View Post
    I found it helpful too - I persoally dont have a problem with wordiness

    What I was confused though, is it true that addiing a linear polarizer in front of a circular polarizer will work? I always thought that LPs wont' work with AF, but from what you appear to say, the CP will "restore" the LP back to a compatible format with AF.
    Yes. Think of it this way. When you stack an LP with a CP, you are actually stacking LP + LP + quarter wave plate.

    The LP + LP stack gives you the variabe ND function. The light passing through this stack is linearly polarised thus not compatible with modern AF and metering systems. The quarter wave plate then transforms the light wave into circularly polarized light, which then makes it compatible with the camera.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

  4. #24
    Senior Member zac08's Avatar
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    Default Re: Two CPLs = Zoomable ND?

    Quote Originally Posted by roygoh View Post
    Yes. Think of it this way. When you stack an LP with a CP, you are actually stacking LP + LP + quarter wave plate.

    The LP + LP stack gives you the variabe ND function. The light passing through this stack is linearly polarised thus not compatible with modern AF and metering systems. The quarter wave plate then transforms the light wave into circularly polarized light, which then makes it compatible with the camera.
    Hmmmm... I may try this.

    After-all, I bought one LP mistakenly previously... so now I have 1 LP and 1 CPL... time for trials. Thanks
    Michael Lim
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  5. #25
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    Default Re: Two CPLs = Zoomable ND?

    Hmmmm think I should point this out:
    A circular polarizer is simply a linear polarizer with a quarter-wave retarding plate behind it.
    The quarter-wave retarding plate "messes" up the polarized light so that it's not polarized anymore.

    Hold it: I see confusion coming up..
    1) light moves in all directions (very simplified explanation)
    2) a polarizer moves (at maximum effect) allows light from only 1 direction, giving polarized light. The unwanted (reflections) are gone, removed.
    3) light particles move in only 1 direction screwing up the AF system because essentially the beam splitter in the AF system is another polarizer. If the light is polarized, you can't split light, thus AF can't work properly
    4) The quarter wave retarding plate essentially "rescramble" the polarized light so that it is no longer polarized

    It sounds funny that a polarizer doesn't give polarized light, but you must remember the light has already been "filtered" the way you want by the first layer. The evil reflections removed, so it doesn't matter if it's polarized or not when it leaves the 2nd plate.

    For this reason however, CPL are supposedly less effective than LP, but more expensive due to the birefringent material they use for the retarding plate.

    Applying this knowledge, the CPL must always be the last one in a stacked configuration, as in the one nearest to the body's AF system.
    Last edited by unseen; 2nd November 2006 at 05:42 PM.

  6. #26
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    Default Re: Two CPLs = Zoomable ND?

    Oh yeah, laws of physics.. if you apply 2 polarizers at 90 degrees to one another, you would (in theory) kill 100% of the light. Last I played with the B&W slim polarizer, my impression was that it doesn't work well (not effective). I presume if you use B&W polarizers, you won't see total light cancellation.

    Another thing I just thought of: If you stack 2x CPL, you won't have a very good variable ND filter (as in can't go super dark). can anyone varify this? In theory the 1/4 wave retarding plate would throw a wrench into the idea of polarizing a polarized light. You need a LP and a CPL for it to work properly (better).

  7. #27
    vince123123
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    Default Re: Two CPLs = Zoomable ND?

    Ah, time to find a good quality 77mm LP to try liao

  8. #28
    Senior Member azul123's Avatar
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    Default Re: Two CPLs = Zoomable ND?

    Quote Originally Posted by zac08
    Hmmmm... I may try this.

    After-all, I bought one LP mistakenly previously... so now I have 1 LP and 1 CPL... time for trials. Thanks
    Would really like to know what you found out with the LP/CPL stack, if got pictures better still.

    ../azul123

  9. #29

    Default Re: Two CPLs = Zoomable ND?

    Quote Originally Posted by unseen View Post
    Hmmmm think I should point this out:
    A circular polarizer is simply a linear polarizer with a quarter-wave retarding plate behind it.
    The quarter-wave retarding plate "messes" up the polarized light so that it's not polarized anymore.

    Hold it: I see confusion coming up..
    1) light moves in all directions (very simplified explanation)
    2) a polarizer moves (at maximum effect) allows light from only 1 direction, giving polarized light. The unwanted (reflections) are gone, removed.
    3) light particles move in only 1 direction screwing up the AF system because essentially the beam splitter in the AF system is another polarizer. If the light is polarized, you can't split light, thus AF can't work properly
    4) The quarter wave retarding plate essentially "rescramble" the polarized light so that it is no longer polarized

    It sounds funny that a polarizer doesn't give polarized light, but you must remember the light has already been "filtered" the way you want by the first layer. The evil reflections removed, so it doesn't matter if it's polarized or not when it leaves the 2nd plate.

    For this reason however, CPL are supposedly less effective than LP, but more expensive due to the birefringent material they use for the retarding plate.

    Applying this knowledge, the CPL must always be the last one in a stacked configuration, as in the one nearest to the body's AF system.
    The 1/4 wave plate doesn't mess up the polarized light. It just allows a circularly polarized light pass through. Linearly polarized light in any direction will be cut by half. How it works is that for a certain wavefront, the light that hits the first linear polarizer will allow light of that polarization to come through, 1/4 wave later, another polarizer will cut whatever light that is not polarized in the orthogonal direction.

    The only case where the light can fully pass is when the light is circularly polarised where the E-field has rotated by 90 degrees after 1/4 wave also. The linearly polarized light which will come through will be attenuated because it will not have an orthogonal component after 1/4 wave and since the light is not coherent, only half of the light can pass through.

    When light scatters, it becomes arbitrarily polarised. However, if light is reflected off a medium transparent, depending on the material, the reflection and transmission will be different for different polarization angles. So the light reflected from vertical glass panels at 45 degree will be vertically polarized and the rest of the light is transmitted through.

    The reverse effect is true because in the mirror box, the central part of the mirror is semi-transparent for the light to reach the AF sensor. This mirror is horizontal, so horizontally polarized light will not be able to transmit through and reach the sensor.

    So you will find that for circular polarization filters, reflections off plane surface is reduced but not totally eliminated. The only total elimination is when the light is polarized in the opposite rotation.
    Last edited by lsisaxon; 3rd November 2006 at 12:34 AM.

  10. #30
    Senior Member zac08's Avatar
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    Default Re: Two CPLs = Zoomable ND?

    Quote Originally Posted by azul123 View Post
    Would really like to know what you found out with the LP/CPL stack, if got pictures better still.

    ../azul123
    Gotta give me some time... I'd need to get a tripod first.

    And I will probably try it out this weekend. Will update on this thread.
    Michael Lim
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  11. #31
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    Default Re: Two CPLs = Zoomable ND?

    Need to check for colour balance also, especially at near 90 degree setting. The polarizers may not be totally neutral, and may have varying effect on different colours. Not sure if custom WB can be used if this turns out to be a problem.

    Hope to see someone post their test results.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

  12. #32
    Senior Member azul123's Avatar
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    Default Re: Two CPLs = Zoomable ND?

    Quote Originally Posted by zac08
    Gotta give me some time... I'd need to get a tripod first.

    And I will probably try it out this weekend. Will update on this thread.
    Thanks in advance for your efforts, me trying to get a LP also... so far no hits on my WTB post.

    ../azul123

  13. #33
    Senior Member zac08's Avatar
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    Default Re: Two CPLs = Zoomable ND?

    Here is a very simple test in which I tried to stack 1 x CPL and 1 x LP onto my 50mm f1.8 lens against a fixed lighted object.

    #1 - 0.625 seconds f11 (no filters at all) this is for comparision


    #2 - 2.5 seconds f11 (CPL + LP)


    #3 - 5 seconds f11 (CPL + LP) I turned the LP approx about 10 to the right from the last position


    #4 - 10 seconds f11 (CPL + LP) I turned the LP approx about 10 to the right from the last position
    Michael Lim
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  14. #34
    Senior Member zac08's Avatar
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    Default Re: Two CPLs = Zoomable ND?

    #5 - 18 seconds f11 (CPL + LP) I turned the LP approx about 15-20 to the right from the last position


    After that, the next 10 degrees gave me a black VF.


    #6 - 1 second f11 (ND 2)


    #7 - 2 seconds f11 (ND 4)


    #8 - 3 seconds f11 (ND 2 stack on ND 4)
    Michael Lim
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  15. #35
    Senior Member zac08's Avatar
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    Default Re: Two CPLs = Zoomable ND?

    Btw,

    all images are 100% crops of the orginal image.



    This is illuminated by a simple table-top Ikea light (which may have given it a orangy yellow cast to the picture)

    As you can see from the crops, there are certain shifts in colouration when I turned the LP. So while it might be a good tool to have for changeable ND filtration, there may be some limitations in terms of colour cast. Mebbe we could remove this problem when we shoot RAW.

    Hope this simple test helps.


    Cheers,
    Last edited by zac08; 4th November 2006 at 01:55 AM.
    Michael Lim
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  16. #36
    Senior Member azul123's Avatar
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    Default Re: Two CPLs = Zoomable ND?

    Appreciate your sharing with us the results.
    ../azul123

  17. #37
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    Default Re: Two CPLs = Zoomable ND?

    Quote Originally Posted by azul123 View Post
    Dunno how much, never seen one yet.

    ../azul123
    77mm 60SGD.

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