Two Polarizing Filters = One Variable ND Filter?

Discussion in 'Beyond the Basics' started by SueMe82, Jan 8, 2008.

  1. SueMe82

    SueMe82 TPF Noob!

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    I've read on the net (here) that if you use two polarizing filters together you can make a "infinitely variable ND Filter", i was just wanting to know have any of you tried this and if so how does it compare to using a higher strength ND Filter?
     
  2. CPayton

    CPayton TPF Noob!

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    Ya, I tried that. It does work but with the filters I have (not very expensive) it tended to leave a purplish cast at higher densities. I guess it would be possible to correct it using a whibal or other white balancing method. I'd probably use it in a pinch but would use ND filters if available.
     
  3. Sideburns

    Sideburns TPF Noob!

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    It works. My photography teacher actually said that if we had two polarizers to try it...

    You can put them 90 degress to each other and get a pretty much black viewfinder....and do exposures for quite some time in bright daylight.

    I haven't tried it though...
     
  4. usayit

    usayit No longer a newbie, moving up!

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    I purchased a UV filter and two polarizing filters. Popped out the polarizing glass and the UV glass from their filters and inserted the polarizing glass into what was once the filter ring for the UV glass. I then screwed it too the rotating filter ring of the 2nd polarizing filter. Having two rotating filter rings made it difficult to adjust and make the whole setup a bit too thick (possible vignette).

    It works like a champ....

    A while back, one of my buddies didn't believe me so I made a quick video

    http://www.silveredemulsions.com/temp/VarND.mp4
     
  5. usayit

    usayit No longer a newbie, moving up!

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  6. Garbz

    Garbz No longer a newbie, moving up!

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    I also suggest you get one linear and one circular polariser. Depending on the design the linear one on the outside will allow you much greater consistency and the true ability to get 100% blackness.
     
  7. Josh66

    Josh66 Been spending a lot of time on here!

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    Cool, this sounds like something I have to try.
     
  8. jstuedle

    jstuedle No longer a newbie, moving up!

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    The only issue with this solution is the linear poly will prevent the AF system on most if not all cameras from working. AF is the reason circular poly's were made.
     
  9. Helen B

    Helen B TPF Noob!

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    That's why Garbz suggested that the linear should be at the front. The light arrives at the linear polarizer, then passes through the circular polarizer - you can consider a circular polarizer (in simplistic terms) as being a linear polarizer followed by a depolarizer.

    Two circular polarizers would not work unless the front one was the wrong way round, nor would a circular in front of a linear.

    Best,
    Helen
     
  10. usayit

    usayit No longer a newbie, moving up!

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    Really? Mine works and it is made from two circular polarizers... I have to admit, I don't have an scientific understanding of polarizers. All I know is that I figured it out via experimentation and set on a DIY project. Perhaps when I popped out one of the polarizers from its rotating filter ring, I flipped it around when I remounted on the UV's ring. I swear that I had them both facing forward when I was "playing" with them at the counter.

    Garbz suggestion of using a linear polarizer has me very curious. I think I'll have to find one and give it a shot. I'm not concerned with how it affects AF as the camera that the "VariND" filter is used on doesn't have AF.
     
  11. Helen B

    Helen B TPF Noob!

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    Here's a test.

    If your first circular polarizer is mounted with the delay plate in front of the polarizer it will have no effect as a polarizer - ie it won't affect the sky or reflections as it is rotated. This is, in fact, the best way of making a variable ND that has no polarizing effect. One circular polarizer backwards, then one forwards.

    Best,
    Helen
     
  12. RacePhoto

    RacePhoto TPF Noob!

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    So what you are saying, is when I used two circular polarizing filters for a test last Summer, I was only using one? :lmao:

    I understand what you are saying and the why, I just don't see how the filter is polarizing and unpolarizing and making any difference if there's only one? Wouldn't it remove the effects, and make the filter do nothing in that case as well.

    But you do make a good point. Now I have to go test the theory and see if I was just wasting time and money by using the second filter. :er:

    Good news is that old linear 58mm filter from the 60s is still in a box somewhere, and my new 100mm lens is a 58mm threading. Perfect for side by side testing against two 77mm circulars on a different lens.
     

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