What to consider when buying a polarizing or IR filter?

Discussion in 'Photography Beginners' Forum' started by thereforeiamx, Apr 27, 2008.

  1. thereforeiamx

    thereforeiamx TPF Noob!

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    I have some basic questions about polarizing and IR filters! I want to buy both, but the question of what to consider confuses me.

    1 - What's the difference between a linear and a circular polarizer? Pros/Cons?
    2 - Will the polarizer/IR filter I buy fit/work on ANY dSLR lens as long as the openings are the same circumference? [say, 55mm]
    3 - For the IR filter, how will factors, such as aperture/exposure time/ISO affect the tones of the picture that results?

    :p
     
  2. Helen B

    Helen B TPF Noob!

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    A circular polarizer is just a linear polarizer followed by what is effectively a depolarizer. This means that the camera sees 'depolarized' light even though the incoming light has been linearly polarized. Linearly polarized light can fool autofocus or exposure meters, so by depolarizing the light the autofocus or exposure system can function correctly. There are no real disadvantages to circular polarizers in most circumstances.

    Here are some previous threads on polarizers:

    Thread 1

    Thread 2

    Thread 3

    Very wide angle lenses may need thinner filters - probably without front threads - to prevent the filter ring from appearing in the corners of the frame. Otherwise a 55 mm filter should fit all lenses with 55 mm threads.

    Are you asking about IR filters with film or with digital?

    Best,
    Helen
     
  3. thereforeiamx

    thereforeiamx TPF Noob!

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    IR for digital.
    p.s. you work at a very nifty place :)
     
  4. Garbz

    Garbz No longer a newbie, moving up!

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    IR some extra consideration needs to be thought of. How steep do you want the cut-off? Hoya R72 = 720nm Wratten 87 = 750nm Hoya RM90 = 900nm cutoff.

    All your exposure relationships stay the same. THe problem with IR for digital is that your fighting one filter that blocks visible light against the filter that blocks IR light. So whereas I normally shoot 1/800 f/16 ISO800 during the day, I would need to adjust this to well above 10" with the same settings on my D200. The problem is less pronounced for other cameras though.
     
  5. thereforeiamx

    thereforeiamx TPF Noob!

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    stupid question: I'm assuming that the lens cap will still fit on the lens even with the filter on?
     
  6. usayit

    usayit No longer a newbie, moving up!

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    It should....
     
  7. rubbertree

    rubbertree TPF Noob!

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    Depends on the type of filter. I have a B&W polarizer and no, the cap does not fit on with it.
     

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