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This question already has an answer here:

I'm looking into filter systems and don't know whether I should get a filter kit with multiple filters or a variable filter.I've had bad experiences with a CPL filter on my wide angle lens and I'm not sure if a variable ND filter would work well on a wide angle lens. Any recommendations?

marked as duplicate by dpollitt, Caleb, scottbb, inkista, Dan Wolfgang Jan 4 '17 at 16:17

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    I basically asked this same question, but after buying a variable ND - Why are my results with a variable neutral density filter poor?. This is probably a duplicate of that question. – dpollitt Dec 31 '16 at 1:22
  • @dpollitt That looks like what I was expecting. One ND filter of decent quality costs more than some variable filters. I knew there was a catch to it. Thank for the info! I'll not buy a variable filter! – Chris Dec 31 '16 at 1:46
  • I started to answer this and then I realized that the answer is even more complicated quantum mechanically than I thought. Two polarizers will allow you to adjust the 'extinction' point where little light gets through. If you add a third polarizer (sky, water, etc.) the extinction point can/will change in those areas. Lots of literature on this- google 'three polarizers' or see the explanation at informationphilosopher.com/solutions/experiments/… – BobT Jan 2 '17 at 19:01
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Variable ND filters are made up of two polarizing filters stacked on top of each other. The angle of polarization between the two is what determines the density of the system.

Polarizing filters don't work so great on wide angle lenses because those filters have the most effect when the subject (sky?) is 90° to the sun. Since a wide angle lens covers much more than 90°, the sky is darkest at that angle and then gets lighter away from that angle. The effect can be very unsatisfactory, to say the least. Regular ND filters do not have this effect. However, since variable ND filters are composed of two polarizing filters, they will have the same effect with wide angle lenses, though you may feel free to use any regular ND filter on any wide angle lens of your choice.

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    I'm not sure this addresses variable ND filters though. – dpollitt Dec 31 '16 at 1:19
  • Variable ND filters are polarizing filters. – Michael C Dec 31 '16 at 2:22
  • Thanks for the edit and comment. I have edited my answer so that it is (hopefully) more concise. – BillDOe Dec 31 '16 at 19:46
  • I didn't get the 'the subject is 90° to the sun and a wide lens covers more than 90°' thing.. – user152435 Jan 1 '17 at 3:12
  • @user152435, if you are shooting a subject and the sun is in line with either of your shoulders, then it is 90° from the subject. – BillDOe Jan 1 '17 at 21:42

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