Interesting ND filter, Anyone tried?

Discussion in 'Photography Equipment & Products' started by Manlatiger, Jan 23, 2009.

  1. Manlatiger

    Manlatiger TPF Noob!

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  2. Pure

    Pure TPF Noob!

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    Judging by the crappy English, and it's from china, it's probably low quality glass with a even crappier coating.
     
  3. dEARlEADER

    dEARlEADER TPF Noob!

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    i especially enjoyed the "high quality professional photography" seal of endorsement on the add...

    $200 bones??

    i dunno.... it's unproven....

    i'd rather waste my money on the more affordable cokin p series....

    if you buy it let us know how it works out..
     
  4. Joves

    Joves No longer a newbie, moving up!

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  5. Eldrich

    Eldrich TPF Noob!

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    if you just stacked two polarizing filters, you'd get the same control. you might have AF issues if you're not using circular polarizers, but as long as there are no strongly polarized light sources (i.e. reflections or sky at 90 degrees to the sun) they would behave just like a ND filter with variable control (spinning one polarizer with respect to the other) and since you already probably own one polarizer, a second one would be much cheaper than the filter mentioned in the OP.
     
  6. Eldrich

    Eldrich TPF Noob!

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    actually, now that i think about it, as long as you kept the circular polarizer as the inner filter, that is the filter just before the light entering the camera, you would not have any AF problems. So you could get a cheap regular polarizer for the second one to go on top of the circular polarizer and have a very cheap variable ND filter, as long as you were okay with getting the effects of polarizing filters as well.
     
  7. dEARlEADER

    dEARlEADER TPF Noob!

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    you will most likely introduce vignetting when stacking polarizers...

    plus two polarizers only gets four stops.... whereas the cheap p.o.s. listed would give you up to 8 stops

    or make the 8 EASY STOPS!!
     
  8. Joves

    Joves No longer a newbie, moving up!

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    Supposedly easy stops. :lol:
     
  9. Eldrich

    Eldrich TPF Noob!

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    yes, stacking filters may give you vignetting, but I mentioned it in response to someone saying they would stack ND filters.

    and actually, using two polarizing filters you could go from 1 stop, all the way to having absolutely no light making it to the camera. That is as long as the first polarizer was not a circular polarizer. If it was a circular polarizer, then you could only take it down 2 stops I think, not 4 (?) how do you figure 4?

    The way I see it, with completely random polarization, the first filter can polarize all light say, vertically, which would effectively be 1 stop, then if the second polarizer were oriented 90 degrees to the first, no light would get through. And as I said, if the first filter were a circular polarizer, then it would circularly polarize the light, and the most stop you could get from the second polarizer would be one more stop.
     
  10. tirediron

    tirediron Watch the Birdy! Staff Member Supporting Member

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    Interesting theory; not exactly sure I understand the physics you're talking about however.
     
  11. Eldrich

    Eldrich TPF Noob!

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    Basically, if you imagine light traveling directly at you, the electric field of that light can vibrate in two different polarizations (for instance, horizontally or vertically), or it can be in a combination of the two different polarizations. In general, random light will be a random combination of the two directions.

    A polarizing filter, if oriented in the horizontal direction, will block all light that oscillates in the horizontal direction. If you were to rotate the polarizer by 90 degrees, it would block all vertically polarized light.

    So a polarizer will work as a 1 stop ND filter on completely randomly polarized light, it will block on average, half of the light.

    Since the polarizer blocks all light that is horizontally polarized, all light that goes through the polarizer is polarized vertically. Then, the second polarizer can be oriented to block any amount of the light which you want it to. If the second polarizer is aligned the same way as the first it would let all light through, because all of the light that made it through the first is already polarized vertically. If you then rotated the second polarizer, you would see things get darker until the two polarizers were at 90 degrees to one another and then since the first polarizer only let vertical light through, the second polarizer would be oriented to only let horizontal light through, and so none would get through.

    since light is a quantum system, there is a gradation between the two different polarizations, so you can get anything between half of the original light, to no light going through.

    This website has a description of cross polarization:
    Light and Polarization

    but really, any optics book or physics book that discusses polarization will address the same thing.

    If you want I can describe my comment about the circular polarizers having a limitation to 2 stops of ND, but only if you're interested, it would take a little more explanation.
     
  12. tirediron

    tirediron Watch the Birdy! Staff Member Supporting Member

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    Hmmmm... have to give that a try.
     

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