Orquid "Phoenix"

Orquid "Phoenix"

by ceinmart

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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.

So the question: why should I use an ND filter to achieve a darker image at the input, when I can use 2 polarizers instead and rotate them to exactly as dark an image at the input as I want?


Also asked by Julien Gagnet

Is it possible to use two polarised filters to create a variable ND filter?

I was reading that by attaching two polarised filter we could create a variable ND filter.

Has anyone done this? How was this done? Any drawback (colour cast, quality...)? What would be the strength in light filtering of such filter?

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7 Answers

up vote 21 down vote accepted
  • Polarizers are often more expensive than ND filters and you need two of them.

  • Stacking two filters can cause vignetting with wide lenses.

  • You have an extra glass surface with two polarizers which can cause flare and potentially loss of contrast/sharpness.

  • This arrangement can cause colour shift toward yellow (but so some ND filters).

  • Extreme wide angle lenses will exhibit uneven darkening due to the difference in incidence angle across the polarizers.

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And stacking filters will decrease image quality, as it's another piece of glass light has to go through before reaching the sensor –  t3mujin Jun 2 '11 at 11:47
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Also the effect the polarisers has to create the darkness will not work evenly the wider the angle of your lens gets, in extreme cases causing a cross-like pattern of light and dark –  Dreamager Jun 2 '11 at 12:01
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The short answer is that yes, you can do this. Just stack two polarizers, and when you rotate them relative to one another, the transmission will vary.

Make sure that the polarizer in front is either:

  • linear, not circular, as the latter essentially un-polarizes the light as it exits the filter.
  • a reversed CPL (but now the threads won't line up).

The point is that the light exiting the first filter needs to be polarized. The second polarizer will need to be a circular polarizer if you want autofocus.

I just tried this (two CPLs, the one in front reversed) and the attenuation seems to vary between one CPL's worth (1.5-2 stops) to nearly black. I did get a strong purple tint when I was approaching max attenuation.

Be aware that you'll still get all the effects of a polarizer as well.

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Normal unpolarized light rays have many different "orientations". A polarizing filter only lets through light with a certain "orientation" and proportionally filters out light rays with different orientations. The further the orientation is away from the orientation of the polarizer, the less light makes it through, up 90 degrees where no light makes it through.

If you stack two polarizing filters at right angles, there is no orientation of light that can make it through both filters so the result is zero light transmission. If you vary the angles (most camera polarizing filters rotate to allow this) so they're not quite at 90 degrees you'll let a very small amount of light through, and thus get an ND effect allowing long exposures ect.

The best thing about doing this is that you can vary the strength of your ND filter. The only thing you need to do is get two filters of the same size and make sure the front most polarizing filter is not a circular polarizer.

A non-circular polarizer doesn't mean it's square! Just that after filtering out all but a certain orientation of polarized light, the filter mixes up the orientations of the light coming out the other side. This is done because polarized light with only one orientation messes with the camera's AF.

The only downside of this is that stacking filters can cause vignetting with wideangle lenses.

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In addition to what @Matt Grum gave as reasons:

A polarizer will reduce reflections, while a Neutral Density filter will not. You might want to include reflections in your images.

A Neutral Density Filter optimally does not change the hue or color of the scene at all.

You may not want to darken the sky, and a circular polarizer turned the correct way will darken and emphasize the sky, while a ND filter will darken the entire scene.

Neutral density filters are offered in many different styles. Graduated Neutral Density filters are probably the most useful, where you only want to darken half of the image or a portion of the image. They also provide different transition amounts, with hard transitions or soft transitions.

ND Filters have various transmittance values. You could potentially have a ND filter that far exceeds the maximum light filtering ability of any two polarizing filters.

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Your statement that "you could potentially have a ND filter that far exceeds the maximum light filtering ability of any two polarizing filters" is not true, but I won't down-vote your answer, because I appreciate your help. You should fix your answer though. –  Richard Rodriguez Jun 2 '11 at 16:07
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@RiMMER Polarizing filters don't work perfectly, even when turned at 90 degrees to each other some light will pass. You could view a sheet of steel as a very strong ND filter. This filter will far exceed the maximum light filtering ability of any two polarizing filters. –  Matt Grum Jun 2 '11 at 16:41
    
@RiMMER - I stand with my original answer :-) –  dpollitt Jun 2 '11 at 18:40
    
@RiMMER - I have used both a stacked polarizer, and really strong ND (ND400, allows 0.25% of the incoming light to pass) filters - the ND filter blocks FAR more light. –  Fake Name Jul 1 '11 at 3:13
    
Maybe machine vision polarisers are much more efficient than photographic ones, but two of my linear filters crossed will pass 0.0045% light. Much closer to the steel sheet than the most effective ND filter you can find. This is not a recommendation from my part to use the filter stacking instead of ND filters btw :) –  Michael Nielsen Feb 22 '13 at 18:25
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Speaking from my own experience, I would rather carry different ND filters that combine two polarisers. I stopped using polarisers as I didn't like the way they affect the colours and saturation. I guess two polarisers together will affect the colours even more. Anyway, good luck with your experiment

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I just got a Singh-Ray Vari-ND filter for taking longer duration images. It is essentially what you want to do in a commercial setup. The paperwork with it clearly warns of the color effects mentioned above that you may see on some lenses.

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Yea, beware color casts when stacking dense filters. –  Shizam Oct 31 '10 at 4:29
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A properly rotated polarizer can reduce reflection, but two polarizers with perpendicular axis just reduce the amount of light passes, despite the polarization of the light. This is because light, as a transverse wave, has only two axis of polarization. If both directions are reduced by the same fraction, the result is just a reduction of the overall intensity.

So two such polarizers in theory is identical to a ND filter. But because of the practical reasons mentions by others, you may want to choose just a ND filter.

EDIT: Well, the real reason that light only has two direction of polarization is that photons are massless. If photons are massive, it will have an extra massive polarization. Just so you know~~ :-)

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