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This question already has an answer here:

I know what an ND filter does. I know what a polarizer does. I also know what two polarizers stacked together and rotated properly do. I also know that two polarizers are not the best solution. My question is, how variable ND filters works in theory. What is difference theoretically between good variable ND filter and two polarizers properly stacked together and rotated. Thank you

marked as duplicate by mattdm, MikeW, Caleb, Paul Cezanne, Itai Oct 29 '13 at 3:00

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    As far as I know variable ND filters are all implemented using two polarizers so there's no difference (except that you can't separate the two filters to use as regular polarizers). – Matt Grum Oct 28 '13 at 16:48
  • The first sentences of this question are copied verbatim from the question I marked as duplicate. Something is fishy here. – mattdm Oct 28 '13 at 18:06
  • yeah, that's weird... – Paul Cezanne Oct 28 '13 at 18:21
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    I voted NOT to close since 2 polarizers stacked is not exactly mechanically the same as a special purpose variable ND filter (which is made of 2 polarizers stacked). Admittedly, the whole point of the question escapes me, but it is interesting. – Paul Cezanne Oct 28 '13 at 20:14
  • @PaulCezanne -- but that's what the other question asks... – mattdm Oct 28 '13 at 23:11
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Matt Grum is right but I can think of one difference.

Variable Polarizers have a well known problem with very wide angle lens. You get strange banding in your image since the light travels so obliquely through the filters.

Two polarizers could have a greater gap between them which could exacerbate this effect.

  • Is the banding from the gap or from the light from wider field changing incoming angle across the frame in a more pronounced way? (as seen in threads like this: dpreview.com/forums/post/31540057 ) – Patrick Hughes Oct 28 '13 at 20:21

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