Filter Question

Discussion in 'Photography Beginners' Forum' started by thenikonguy, Sep 2, 2009.

  1. thenikonguy

    thenikonguy TPF Noob!

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    I bought some old gear a few weeks back, and there was a very nice polarizing filter on one of the lens.. all well and good.. the filter has 2 rings to it, theres a ring that has the threads to attach it to a lens.. but then, there is also a outter ring, that spins.. the filter is the same color all over, not darker on 1 half or anything.. so why would the outer ring spin??
     
  2. Plato

    Plato TPF Noob!

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    You're not entirely clear but if the glass is held by the outer ring, that's how all polarizers work, always have and always will be. You rotate the filter to change the degree of reflection that's eliminated.
     
  3. chammer

    chammer TPF Noob!

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    yah..sounds like a cpl. i just bought one myself, and i get how to make it work...but what i dont get is how it works. it looks the same all over, but can someone explain how turning it works or point to something that explains it in simple terms?
     
  4. thenikonguy

    thenikonguy TPF Noob!

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    thats what i thought too.. but I guess its the "how it works" that I don't get..
     
  5. Plato

    Plato TPF Noob!

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    Think of throwing a bazillion frisbee toward venetian blinds. Depending on the angle of each individual frisbee and the angle of the blinds, it might slip through and it might bounce off. Now, take this a step further and consider that light that's reflected from non-metallic surfaces is a half-bazillion frisbees oriented in the same direction. Rotate your window (with the blinds) so as to block those frisbees and you'll appreciate what a polarizer does.

    Circular polarizers are a bit more confusing. The front surface of the glass does exactly what I described in the previous paragraph. However, polarized light (all frisbees in the same direction) play havoc with autofocus and light metering mechanisms. The back surface of the glass of a circular polarizer therefore "circularizes" the light (randomizes it, depolarizes it) so that autofocus and light meters function properly.

    By the way, automobile fenders are a non-metallic surface - paint is non-metallic.

    With the old polarizers (linear polarizers), you could use two and, depending on the relative positions of the polarizing lines, have an infinitely variable neutral density filter. You can't do that with a circular polarizer because of the circularization.
     
    Last edited: Sep 2, 2009
  6. phifer8390

    phifer8390 TPF Noob!

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    Mine has this also and if I am not mistaken it is so you don't over tighten it on the lens. I really see that as being the only thing it could be.
     
  7. Plato

    Plato TPF Noob!

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    No, no, a thousand times no. You need to rotate the polarizer so that it does what you want it to do. By the way, it's a good habit to always rotate clockwise so that you don't inadvertently unscrew the filter.
     
  8. Dao

    Dao No longer a newbie, moving up!

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    Technically, polarizer filter is used for selectively filtering light. As the name imply, it filter polarized light.

    Light wave normally oscillate in many direction. But in certain condition, it only oscillate in one plane. And we called it polarized light.

    Reflected lights are usually polarized. Hence, polarizer filter can be used on reduce the amount of light from a reflected surface such as the surface of a pond or glass.

    Lights can be polarized by scattering. The blue sky was the result of that. However, some of the partially polarized light will make the sky kind of wash out. So block those light from entering the camera can make the sky deeper blue.

    As for why circular and rotatable, since you want to rotate the polarizer so that the invisible slits are perpendicular to the polarized light oscillating plane. In that way, it will stop those light from passing through.
     
  9. phifer8390

    phifer8390 TPF Noob!

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    Ok... never mind my newbieness. I'll have to play with it and see the difference.
     
  10. EleanorW

    EleanorW TPF Noob!

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    lol you're further along then me - had my cpl for a week and it's still in it's case :D
     
  11. Plato

    Plato TPF Noob!

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    Pretty close but not exactly correct. Probably easier to explain if you first read my earlier post with the frisbee analogy.
     
  12. phifer8390

    phifer8390 TPF Noob!

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    So I checked it out and I won't really be able to tell by just looking at it, but after rereading the post by plato above about the frisbee I understand the concept a little better. I also understand the more technical post by Dao because I can just relate it to going fishing.
     

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