Adjustable ND Filter

[bmpress]

I found a new Polaroid filter that claims to be adjustable over a wide range and just ordered one. If it works well, it will eliminate the need to carry several strengths. They say that by rotating the filter it's density changes. Have you used one and is it a good way to go? I believe it works by using polarizing filters rather than darkening materials. Any information would be appreciated. Although it sounds too good to be true, it is cheap enough to try one.

[Bob Atkins]

In principle it's fine as long as it's built right and used high quality materials. I suspect the cheap ones may introduce a color cast.

You could do it yourself of course just using two stacked polarizers. Crude, but might work OK over a limited range.

I haven't used one myself, but I might be testing one in the near future. I have been contacted by someone who is making one (not a cheap one!) and it would be interesting to see how well it works.

[bmpress]

I'll send some test shots when I get the filter.

I'm not sure that I understand how two polarizers do the job since a single one would only cut the exposure by one-two stops.

[klindup]

Think of the polarising filter as a bit like a picket fence.  Throw a pile of sticks at it and only those parallel with the fence posts will get through the gaps.  Take two fences and rotate one so that its posts are at right angles to the first and you have a series of squares and nothing will get through.  The light hitting your filter is composed of rays with different planes of vibration (if you see what I mean) only some will get through.  Add another filter rotated so as to make its plane at right angles to the first and nothing will get through.    I could explain it more easily with drawings on paper and it would become obvious why the filter works. No doubt Bob can come up with a better explanation than this :-)
Ken

[Bob Atkins]

Polarizers can be very tough to explain because when you look deeply enough at how they work you have to invoke quantum principles. - see http://www.lanl.gov/science/centers/quantum/light.shtml ot perhaps http://www.informationphilosopher.com/solutions/experiments/dirac_3-polarizers/ for examples of how the simple non-quantum explantions can fail.

However the picket fence analogy works pretty well at explaing what goes on with linear polarizers when you only have two of them. Add a third and you get into trouble!

[bmpress]

Thanks for these references, Bob. After reading them several times I have a better understanding of polarization and quantum mechanics. It brings to mind my first exposure to tunnel diodes in school, where they talked about the probability of an electron jumping the gap. It was mind-boggling at the time, but since the theory proved to work, had to be accepted despite the weirdness of it all.

Now I can hardly wait to get the Polaroid product and see how good it is.

Barry

[bmpress]

Bob, I just got the product and, looking through it, can see a very wide darkening towards black as I turn the rim. But now I am in a quandary of just how to test the filter. Any suggestions?

[Bob Atkins]

What I'd do would be to take a series of images of the same subject at various filter settings. Put the camera on a tripod, set the mode to Av, set the ISO to something fixed (don't use Auto ISO), set the white balance to something fixed (don't use auto), then take the shots. I'd be looking for shifts in color as the density is changed. Exposure times may get long at the maximum density setting, so you will probably need the tripod to hold the camera steady.