Stacking ND filters vs. 2 polarisers

Discussion in 'Beyond the Basics' started by quad b, Mar 30, 2007.

  1. quad b

    quad b TPF Noob!

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    When I bought my ND filter a while ago the salesjerk, I mean salesperson, said that instead of stacking ND filters you could stack two circular polarisers and just rotate them to get different levels of density so to speak.

    Now in theory this would work, but I was wonderng if more than one polariser messes about with autofocus?
    Or if anyone has tried this method out and how it goes compared to ND filters?
     
  2. Alex_B

    Alex_B No longer a newbie, moving up!

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    Never tried this myself

    well, it will get pretty dark anyway, so AF might fail you in any case with the ND .. and the stacked polarisers.

    but as I said, I never tried this myself.
     
  3. darich

    darich TPF Noob!

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    It can't work....at least not in the way the salesman suggests.

    A polarizer affects the light as it passes through and hence polarizes it. Putting 2 filters on will mean that less will get through, possibly none depending on the rotation of each one but a Neutral Density filter doesn't affect the light other than reduce the amount getting to the film/sensor.

    So putting on 2 filters that affect the light will definitelty give a different effect when compared to one that doesn't affect the light.

    I'd imagine that focusing may be more difficult if there is much less light getting to the sensor. It may still work in bright conditions but if it's a bit darker then there may not be enough light for the camera to figure things out.
     
  4. fmw

    fmw No longer a newbie, moving up!

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    No, light polarizes itself simply by coming at the lens from a certain angle with the light source. What a polarizer does is filter polarized light leaving the letting the un polarized light pass. It is a filter, just like any other filter. It doesn't polarize light or do anything active.

    What Darich says about the effects of that, however, is essentially correct.
     
  5. darich

    darich TPF Noob!

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    i knew that since it's called a filter......i also know that a red filter doesn't make light red - it simply blocks all light except red - same principle.
    i just didn't say what i meant!!
    :thumbup:
     
  6. Garbz

    Garbz No longer a newbie, moving up!

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    Actually this is exactly what it does. Since light generally travels in all directions around it's axis of propegation, a polariser ensures only a certain angle gets though. Yes the side affect of this is it may block polarised light in certain angles but the light coming out of the filter is as polarised as it gets :D

    I do know what you were trying to say though :wink:
     
  7. shorty6049

    shorty6049 TPF Noob!

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    quadb, where did you get that avatar? i love it.
     
  8. quad b

    quad b TPF Noob!

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    thanks, i got it from the movie "fightclub". i found on the web somewhere.

    has anyone tried two polarisers?
     
  9. hazzayoungn

    hazzayoungn TPF Noob!

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    never tried it on a camera, but we did fiddle with them in physics class

    we took two linear polarizers and stacked them and rotated. (you probably already know this but) polarizers change the way the light moves around http://en.wikipedia.org/wiki/Image:Wire-grid-polarizer.svg

    light is a wave. the polarizer makes the wave two dimensional rather than three. if you stack two polarizers at full potential, the wave will be 1 dimensional ie...a line and no longer a wave. no light will get through.

    theoretically the first polarizer will get rid of the glare and darken the sky like normal. the second will darken all the rest of the light

    but like i said, ive never tried it

    ps...a little off topic, but im a nerd and cant help it :D http://www.colorado.edu/physics/2000/polarization/molecular_view.html

    EDIT: while browsing that page, i found this very informative java applet http://www.colorado.edu/physics/2000/polarization/blocking_light.html

    EDIT part 2: the whole tutorial on polarizers is interesting...for me at least. last post i swear
    http://www.colorado.edu/physics/2000/polarization/
     
  10. fmw

    fmw No longer a newbie, moving up!

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    What I was saying is exactly what I was trying to say. Light is polarized, not because of a filter. It is polarized naturally as a result of its angle of incidence from the source. Filters filter polarize light. They don't create it or polarize it. I do know what you were trying to say, though.
     
  11. Ken_D

    Ken_D TPF Noob!

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    If you looked at a polarizer under a microscope, you would see thousands of microscopic parallel lines. Much like a picket fence. When you rotate it, it only allows light to come into the camera from one direction (angle), blocking the light trying to come in from any other direction (angle). Years ago (before the days of autofocus), they would stack two polarized filters. The one closest to the lens would remain fixed, the other rotated. When the lines in both filters were parallel, you would get around a ND-4. when you rotated the front filter, you could get almost a complete black out when the filters (lines) were at right angles to one another (essentially a variable ND). So your salesperson was right, it can be done, but it would be hard to use your autofocus for the same reason a f/1.4 lens will focus sharper than a f/4 lens. The amount of light reaching the camera's sensor.

    I've had to use this before, I was a medical photographer in the '70's and used the polarizer discribed above to photograph surgical procedures. Due to the light reflecting off stainless steel surgical instruments a polarizer was necessary, and because of the amount of light used it was often necessary to have a variable ND filter.
     
  12. quad b

    quad b TPF Noob!

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    yeah, we tried this in a physics lecture at uni as well.

    i've just been wondering if the outcome is much different to stacking ND filters.
     

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