Making a variable ND filter with circular polarizers

November 13, 2007 by Daniel | Filed Under DIY, Photography 

I read about the Singh-Ray Vari-ND variable neutral density filter and found it really interesting. But then I looked at the price tag… over $300 is a little too much to spend on a filter for my taste.

So I started thinking. Isn’t it basically two polarizers screwed together?

Some basic theory on polarizers:
The basic polarizing filter is the linear one, it does nothing but polarize the light so that it is oscillating in one direction. If you put two linear polarizers over each other with the direction of polarisation perpendicular they will block out all the light. When you rotate one so that they are not perpendicular they will let through varying amounts of light. If they are parallell, you get no effect in the image.

But modern cameras have polarizers in front of their focus and metering sensors, so using a normal linear polarizer can disable those sensors when the light happens to be polarized in the wrong direction. Thats why there are circular polarizers.

Circular polarizers have a quarter wave retarder on the back of the filter, which scatters the light so that it is no longer polarized (or rater it’s circularly polarized). So you get the polarized effect with reduced reflections and deep blue skies etc. but with non-linearly polarized light entering the camera so that it doesn’t mess up the metering.

In this image I tried to show what I mean. The vertical component of the light passes through, and is scattered so that it still oscillates in both directions after passing the filter.

So first I figured that the Vari-ND must use two polarizers, one linear in front of a circular. That would first polarise the light in one direction, then the second would cut out some light depending on the rotation and lastly the quarter wave retarder would “un-linearly-polarize” the light before it enters the camera.

The problem with that is that the image would get a polarized look because of the linear filter in front. After some googling I realised you need two circular polarizers, with the front one put on backwards, to get rid of the polarization in the final image.

So I have this old 52mm circular polarizer that I’m not using much, and I placed an order on another today to try to make my own Vari-ND. Stay tuned!

Update:

I got the new circular polarizer today. It’s a cheap noname polarizer, it cost me about $15. I bought the cheapest one I could find.

After some quick tests it seemed to work OK, but there was a slight blue or yellow color cast when looking through the two filters at polarized light.

Here it is:

I used a small screwdriver to loosen the ring holding the glass in place. The new filter had bigger grooves and was much easier to remove the ring from so I decided to flip the glass in that one (plus it was cheaper so I’m not as careful with it).

Next, I removed the ring and took out the glass. After flipping the glass around I put it back in the filter and fastened the metal ring again.

…

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So there it is, the assembled diy variable neutral density filter!

On top it the modified filter, and on the bottom the normal one. Time to try it out.

After some testing my worries were confirmed. Sometimes there was a blue cast, sometimes a yellow, and sometimes no cast. It seemed like it only affected polarized light. Though except from that, it’s light cutting capablities are as expected.

The light from my laptops LCD screen is polarized, as I found out when I held the CPL up in front of it.

So I used the screen for some testing.

If I held up a filter in front of the screen backwards, I got a cast. First it was all blue.

I rotated it 45 degrees and the cast dissapeared and it looked normal, after 90 degrees it got a strong yellow cast instead.

I tried the same with the other filter, and I could see a very slight yellow and blue cast when flipped, but far less than with the new one. So I would probably have got a much better result with the old one backwards in front.

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I don’t know whats causing the cast, but when I took apart the filter I noticed the pieces of glass were pretty thick and that there was two seperate pieces that were sandwiched together, see closeup below. Maybe the fact that its made out of two separate pieces causes the casts. It feels really cheap (and it was).

The filter has its uses though. For black and white photography it works fine. The slowdown is from about 4 to 15 f-stops. Thats 11 f-stops range.

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In my opinion, if you want flawless performance you should get the real thing. For a makeshift filter thats about 1/10 of the price though, it works pretty good as long as you stay away from the extreme edges of the range, or if you shoot black and white.

I have not yet been able to find a CPL that is cast-free when used backwards, but if anyone has, please put a comment here or mail me so I can try it out.

Filters tested:

• Soligor C-P.L gives a slight cast (the one I used in the back of my vari-nd filter)
• “Nikon CP-15 gives a color cast from blue to sepia”
• “Hoya HMC 58mm PL-CIR has mild yellow/blue cast”

References:

• http://www.fas.harvard.edu/~scdiroff/lds/LightOptics/CircularPolarization/CircularPolarization.html
• http://www.nikonians.org/dcforum/DCForumID8/16174.html
• http://www.meadowlark.com/applicationNotes/Basic Polarization Techniques and Devices.pdf

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