Polarizing filter and sun glasses...

Discussion in 'Photography Beginners' Forum' started by NM Rich, Feb 26, 2008.

  1. NM Rich

    NM Rich TPF Noob!

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    I put a circular polarizing filter on my lens for shooting outside. It just happens that my prescription sunglasses are polarized also. When looking through the view finder, I could barely see what I was shooting. The view had grayish, blurry spots!

    Is there something to put on the view finder to counteract the polarizing effect from my sunglasses? I'm blind without them!

    Thanks!
     
  2. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    I run into that problem in the summer a lot.
    Not much you can do about it. I have clip on sunglasses so I just take them off when shooting with a polarizer, which is often a pain in the @ss.
     
  3. Socrates

    Socrates TPF Noob!

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    Nope. That's the nature of polarization. When you rotate a polarizer one way, it blocks all light waves in a one direction. When you rotate it 90 degrees, it blocks all light waves in the other direction. When you put one polarizer behind another and align them the same way, it's as if you have only one but if you align them 90 degrees apart, you block all light. You need to buy non-polarized sunglasses or, alternately, take them off when you look through the viewfinder.
     
  4. Helen B

    Helen B TPF Noob!

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    Your circular polarizing filter must be allowing a lot of linearly polarized light through. They can never circularly polarize all wavelengths anyway. What brand do you have? I know it sounds like a daft question, but are you sure that it is a circular polarizer?

    In theory your sunglasses should allow half of the circularly polarized light through, no matter what the orientation, unless the light is being polarized by the mirror - in which case your polarizing shades should always show some effect.

    Does the amount of darkening change as you rotate the filter?

    I'm not a big fan of polarized glasses because they affect my ability to see wet shiny surfaces and ice properly.

    Best,
    Helen
     
  5. Digital Matt

    Digital Matt alter ego: Analog Matt

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    Adjust the diopter of your viewfinder so you can see properly without glasses. If your eyes are not too bad you should be able to.
     
  6. NM Rich

    NM Rich TPF Noob!

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    Thanks for the replies!
     
  7. Socrates

    Socrates TPF Noob!

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    ...and, depending on the orientation of the filter, the filter will block that half. I've routinely confirmed that sunglasses are, in fact, polarizing glass by taking two pairs and rotating one in front of the other and confirming that no light comes through.

    This has nothing to do with circular vs. linear. It's simply a matter of how much of what kind of light is blocked.
     
  8. Helen B

    Helen B TPF Noob!

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    I wonder whether or not you understand what a circular polarizer does (or should do).

    A combination of a circular polarizer followed by a linear polarizer behaves differently from a pair of linear polarizers or a combination of a linear polarizer followed by a circular polarizer, if the circular polarizers are the 'right' way round.

    A circular polarizer is a linear polarizer followed by a quarter-wave delay plate with its principal axis at 45 degrees to the plane of polarization of the linear polarizer. In effect this splits the linearly polarized light coming out of the linear polarizer into two linearly polarized portions at 90 degrees to one another. The light then behaves almost as if it were unpolarized in most circumstances (that is the reason you use a circular polarizer for autofocus and autoexposure systems behind a beam splitter).

    A subsequent polarizer will cut out half of the light, no matter what angle the two polarizers are at. This behaviour can be observed - though you need to bear in mind that linear polarizers and circular polarizers are never perfect in their behaviour. This is a different result from observing something through two linear polarizers.

    You can also observe the strange behaviour of a circular polarizer by looking at reflections through it both ways, and rotating it: - look through it the 'correct' way round and it behaves like a linear polarizer, look though it the wrong way round and it has a much weaker effect. If it was a perfect circular polarizer it would have no effect.

    Best,
    Helen
     
  9. Helen B

    Helen B TPF Noob!

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    I did some rough-and-ready experiments last night, and they appeared to support the idea that the problem is caused by too much linearly polarized light getting through the delay plate of the circular polarizer. As I mentioned before, even a high quality delay plate doesn't work perfectly at all wavelengths, so it would be normal to see different effects for different wavelengths (colours). It would be interesting to know which brand of circular polarizer you are using. I'll do some measurements with the ones I have available if I get the chance.

    There could be some depolarization by the ground glass, but that is likely to be very slight. There may be other screens you could use (or make) that would have a greater degree of depolarization, but they would be darker than the one you are likely to be using now, and they could affect the metering system.

    How about getting non-polarizing prescription sunglasses, or switching to disposable contacts and just wearing normal shades? I've worn glasses since I was 11, and I don't like having to take my glasses off to look through a viewfinder, so when I am using a camera with a viewfinder that I can't see properly with my glasses on I switch to disposable contacts.

    Best,
    Helen
     
  10. NM Rich

    NM Rich TPF Noob!

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    Wow Helen, being so new to all of this most of what you said in your posts went right over my head. I do appreciate your thoughts though.

    I think your suggestion of non polarizing prescription sunglasses makes sense for me.
    :cheers:

    Thanks!
    Rich
     

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