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  1. techno79

    techno79

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    It's not entirely camera related but:
    As I understand it, if you have two polarising filters stacked on top of each (aligned to the way they polarise) then they will block about 50% of the light. But as you rotate one of the filters it blocks more and more light until it blocks 100% light when it is 90 degrees. This is exactly what happens with two iMax 3D cinema glasses.

    I needed to do this using something else so I purchased some cheap polarising camera lens filters from ebay but they do not exhibit the same behaviour. There is a subtle change of light but it does not block anywhere near 100% of the light. In fact, it doesn't seem to block any more light than the first filter. The only change is appearance of the colour of the combined lens which turns from a 50% tinted black to a 50% lightly brownish/black tint.

    So my question is, is this normal for camera lenses or are these a problem with these specific ones I've purchased (they were the cheapest ones I could find).

    Or put another way, using two "proper" polarising camera lenses aligned 90 degrees to each other, will they block 100% of the light?

    TIA
     
  2. Mr Perceptive

    Mr Perceptive

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    David
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    Research the difference between linear and circular polarisers, your answer will be in there!!!!
     
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  3. PaulButler

    PaulButler

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    There are two type of polarising filters that I am aware of, linear and circular - I suspect the effect you see (100% light blockage) will only happen with linear.
     
  4. petersmart

    petersmart

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    Not completely - they actually can only block light which is polarised - some light is not polarised so a portion of that light will always get through even though two filters aligned to theoretically block all light will always let some through, even though a much reduced amount.
     
  5. MatBin

    MatBin

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    I normally use a lens cap for 100% light blockage :)
    Arent Circular and Linera polarisers the same (essentially) just digital sensor are confused by Linera, hence we now have 2 versions?
    Matt
     
  6. GreenNinja67

    GreenNinja67

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    Terry
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    I still use an old linear polariser on one of my lenses on my AF digital DSLR.

    Seems to work fine.
     
  7. MatBin

    MatBin

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    Just did a bit of research and one reporter did exactly that because he had used circular filter on his digital camera and found af was "off", images were a bit soft but he wasnt sure if it was because he hadnt spent enough on the circular filter, I guess a cheap one might have that effect.
     
  8. mickledore

    mickledore

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    I used an old laptop screen. Apparently the light from them is polaried. Set it to be pure white.
    I put a polariser on my camera and when positioned correctly everything went black.
    Then I put some plastic cutlery on the laptop screen. Because this is pressed plastic it sets up stress lines in the plastic. This plays havoc with the light patterns and the polarising effects go haywire. This is the shot>>>>
    [​IMG]
    Fork handles
    by Frank Yates2010, on Flickr

    Just a bit of fun on a cold wet afternoon!
     
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  9. GreenNinja67

    GreenNinja67

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    Terry
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    Possible Matt,

    I have a 52mm linear PL and a 77mm CPL, both made by Hoya and I'm unable to see a difference in their use.
     
  10. GreenNinja67

    GreenNinja67

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    Terry
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    I like that Frank
     
  11. mickledore

    mickledore

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    Thanks Terry. Can't claim any originality - I saw the technique somewhere on 't interweb.
     
  12. boyfalldown

    boyfalldown

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    Hugh
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    Do you have problems using AF with a linear PL?


    Thats the way variable ND filters work.
     
  13. GreenNinja67

    GreenNinja67

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    Af works fine in my experience
     
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  14. AndyG123

    AndyG123

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    Can someone explain why you would want to block 100% light from a camera?
     
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  15. Mr Badger

    Mr Badger

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    So it doesn't fog the film! :D :coat:

    Edit: Joking aside, the OP said it wasn't altogether camera related, so it might not be for fitting on the end of a camera lens?
     
    Last edited: Oct 11, 2017
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  16. boyfalldown

    boyfalldown

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    Not quite 100% but a 10 stop ND reduces light transmission by 1000x times and are much used by photographers. You may also wish to block much of the light transmission for various industrial photographs. Wielding being one such use
     
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  17. techno79

    techno79

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  18. AndyG123

    AndyG123

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    Andy
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  19. techno79

    techno79

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    So are polarizing camera lens filters meant to be circular polarising or linear polarizing or both? The ones I bought are probably circular.
     
  20. PaulButler

    PaulButler

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    Either. Linear used to interfere with af systems iirc - whether they do or not now I don't know as I only use a circular polariser ...
     
  21. HoppyUK

    HoppyUK

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    Almost all polarising filters for popular use are circular these days. You're unlikely to find anything else in the average camera shop, but linear polarisers are available, eg from B+W and others. Linear polarisers are supposed to interfere with the AF and/or metering of some cameras, though a few quick tests on my Canons hasn't shown this. Anyway, we have circular polarisers that do exactly the same job with zero penalty, so why not. A circular polariser is just a linear polariser with a quarter-wave plate on the back which circularises or 'de-polarises' the light.

    To block 100% of the light you need a linear polariser in front with either a linear or circular polariser behind, rotated 90 degrees. This will block close to 100%, and the light source doesn't need to be polarised. Edit: if the light source is polarised, such as an LCD screen, then you only need one filter to block the light (see Mickledore's post) - either a circular or linear polariser.
     
    Last edited: Oct 12, 2017
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  22. soeren

    soeren

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    Try turning the front filter (closer to the light source) around so its facing backwards
     
    Last edited: Oct 12, 2017
  23. Steve Smith

    Steve Smith Joe

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    A circular polariser will only let through the light in one direction (say 50%) then randomly 'scatter' it again in all directions. A second polariser will then polarise 50% of that light.

    You need the first polariser to be linear. Possibly the second too, depending on what you want to do.


    Steve.
     
    Last edited: Oct 12, 2017
  24. swanseamale47

    swanseamale47

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    I think 2 linear pola filters used together on digital may give you a blotchy effect, I seem to remember it was something to do with the sensor?
     
  25. soeren

    soeren

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    If you flips the first filter so it faces backwards it will act as a linear polarizer.
     
  26. Steve Smith

    Steve Smith Joe

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    But would it work as a polariser then? The light you would block previously being polarised in one direction would now be random.

    I only have linear polarisers so I can’t check.


    Steve.
     
  27. soeren

    soeren

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    Why shouldn't it? The light coming out of it will be polarized no matter how random it is entering it. The second filter should be oriented as intended.
     
    Last edited: Oct 12, 2017
  28. Steve Smith

    Steve Smith Joe

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    Because if it’s backwards, it would randomise it before polarising. So some light which would have been blocked will now pass due to its new orientation, likewise, some light which would have passed through will now be blocked.

    EDIT: Unless you are just referring to its ability to work with another polariser to reduce the total light going through. In which case, I agree. It just won’t work as we expect a polariser to work.


    Steve.
     
    Last edited: Oct 12, 2017
  29. HoppyUK

    HoppyUK

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    A circular polariser when used back to front does nothing - no polarising effect. It simply acts as a mild neutral density filter. A linear polariser works either way around.
     
  30. soeren

    soeren

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    Yes it polarizes light when flipped backwards but since it's circular any polarized light from reflections will be depolarized first and thus not "turned off. So the light entering the second polarizer is polarized and can be turned off. Checked, works. Now i just need to get access to a PC to upload
     
    Last edited: Oct 13, 2017
  31. soeren

    soeren

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    in advance sorry for really bad IQ

    Circular polariser on camera and normally oriented circular polariser on table

    polfilter skaleret.jpg


    Now the circular polariser on the table is flipped over so the filter thread is facing down.
    Now its doing what Techno79 is asking for

    flippet polfilter skaleret.jpg

    to summarize. the front of the (Circular polarising) filters must face each other in order to do what you asked for in your OP
     
    Last edited: Oct 13, 2017
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  32. HoppyUK

    HoppyUK

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    I think I misunderstood. I was referring to just one CPL used back to front. Apologies :)
     
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  33. soeren

    soeren

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    Thats only due to my lack of english vocabulary and not being able to explain :) back to front, think Ill remember that.
     
    Last edited: Oct 13, 2017
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  34. JackBell

    JackBell

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    A few years ago I was taking photos abroad with my Canon G11, I was wearing polarising sunglasses and when I positioned the camera in the portrait format the rear screen would go blank!! I thought there was a loose wire in the camera and was losing power to the screen when I turned it. Took me 2 days to realise what was happening. :)
     
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  35. soeren

    soeren

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    Problem arises when adding a third polarizer to replicate the experiment in the video, this one must be linear
     

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