CPL question

Discussion in 'Photography Equipment & Products' started by firemedic0135, Feb 3, 2007.

  1. firemedic0135

    firemedic0135 TPF Noob!

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    Hey this may be completely supid but here it goes
    Why does the cpl filter have a line atop and it rotates.I am guessing the line is for reference for something but dont know what.
    Thanks
     
  2. John_05

    John_05 TPF Noob!

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    im horrible at describing things, and im no pro, but a CPL filter rotates so you can help eliminate or increase reflections, and to darken the sky if youre shooting outside. as you rotate it, the sky will get a little "bluer", and reflections will be increased or decreased. they work better depending on the angle your facing the light too, but im not sure about the best angles to use one. hopefully someone who knows more will be by soon to give you more help.

    im not sure if it has any other uses, and im also not sure what the line is for. i have 2 different CPL filters, and neither has a line on it.
     
  3. shingfan

    shingfan TPF Noob!

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    you dont increase reflection with CPL filter....the purpose of CPL filter is to reduce reflection only (purpose of a filter).....the reason with the rotating ring is to allow you to adjust the angle of the filter element with respect to the polarization of the incoming wave (reflection)....hence adjust the amount that is being filtered......usualy it is used to reduce glare of the sun and foliage...so that you get better "blues" and "greens".......CPL is designed for cameras with auto focus as they do not work properly with linear polarized filter.....and link with a little description on advantage of CPL...hope this help

    http://www.offrench.net/photos/articles/polarizing_filter.php

    P.S. only use CPL on outdoor on a sunny day.....if there is not much glare (from direct sun light) in your composition.....remove CPL as CPL reduce the exposure by about half a stop or something (i dont remember the exact number)
     
  4. firemedic0135

    firemedic0135 TPF Noob!

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    Good link:thumbup:
    Thanks for the help guys
     
  5. John_05

    John_05 TPF Noob!

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    if thats the case, then how was i able to increase the reflection in this shot?

    [​IMG]

    using my CPL filter for that shot is what helped to increase the reflection on that window. without it, it didnt look even close to as good as it does in that shot. ill have a look and see if i still have the shots i took without the filter, but all i know is with the filter attached, i was able to get better reflection.

    FYI, im not trying to start an argument. im just curious as to why the shot looks better if a CPL filter wont increase a reflection, even if only a little.
     
  6. firemedic0135

    firemedic0135 TPF Noob!

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    Thats interesting.The reason I got the cpl was due to recent trip to the zoo and the reflections I got off the glass,that and the sunlight beaming down on my lens:D .
     
  7. markc

    markc TPF Noob!

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    It depends on how you rotate it. The light reflecting off the glass gets polarized. You can line up the CPL so that it either lets none of the polarized light through or all of it. Either way, it's still eliminating surrounding light that is not part of the reflection. Some of the light from behind the glass is going to be eliminated, so the reflection will stand out more if it's still there. The reflection is only getting brighter in relation.

    alignment A: 0% reflection, 75% background
    alignment B: 100% reflection, 75% background

    Or something like that.
     
  8. shingfan

    shingfan TPF Noob!

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    from what i understand....filter's job is to reduce....not to increase.....especaillly a glare filter (CPL).....i dont understand how it can increase reflection.....i can see how a CPL does not reduce reflection if you dont rotate it proper...but i dont see how it can INCREASE....doesnt make sense???...also...i dont understand how the CPL increase the reflection in your image?.....btw....CPL's role is not to reduce "reflection"...but to reduce "glare"....they are different....dont mix them up
     
  9. markc

    markc TPF Noob!

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    This way might be better.

    Normal light isn't polarized, so it's like +
    Light reflected off the window will be polarized like |

    If you look with your bare eye, you see it all.

    If you rotate the polarizer to - , then the reflection and part of the light behind the window will get filtered out. The only thing that gets through is the light like - . This is why you lost a stop or whatever it is.

    If you rotate the polarizer to |, then all of the reflection, which is also |, will get through. Part of the background will get filtered, so only the | portion gets through. The - is filtered out. Compared to the naked eye, the reflection is the same brightness, but the background is reduced a stop. Then end effect is that the reflection is relatively brighter.
     
  10. shingfan

    shingfan TPF Noob!

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    +:thumbup:
     

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