ND Filters; Variable? Stacked?

Dan, check out Lee Filters 150mm system, special boot or such used so the filter itself is 150mm square. As I do not need it as I use the 100mm system yet it was developed for the Nikkor 12-24mm lens and will work with the Canon 11-24mm lens. See link below:
http://www.leefilters.com/index.php/camera/system-sw150

Cheers: Allan

Thanks for the info, Allan.

I bought a variable ND filter, then I began finding all sorts of articles slamming them. I went to CIC and searched the forum for "variable filter" and found this thread. As I started reading the thread I found this:

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Originally Posted by Mike Buckley

There have been lots of discussions of the stuff you're asking about. Consider conducting a search and hope there aren't too many threads to wade through.

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Glad I did a forum search and didn't simply post a question. :)

I just finished shooting a bunch of frames with my Tiffen variable ND filter. From min setting to max settings they all looked fine (i.e., uniform) when I handled the filter "normally." However, if I moved the filter to a darker setting and pushed the front lens element to the side I was able to cause the two elements of the filter to mis-align slightly yielding results such as this:

http://i68.tinypic.com/2uq23iw.jpg

Please ignore the watermark it was generated by default. Not my best work. :)

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Originally Posted by paintingwithlight

I just finished shooting a bunch of frames with my Tiffen variable ND filter. From min setting to max settings they all looked fine (i.e., uniform) when I handled the filter "normally." However, if I moved the filter to a darker setting and pushed the front lens element to the side I was able to cause the two elements of the filter to mis-align slightly

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As already stated above, I use a Singh-Ray Vari ND, sometimes stacked with a Singh Ray 5-stop ND.

As soon as you go outwith the marked 'Min' and 'Max' lines the thing doesn't work. You get all sorts of weird results. It's a very precise tool that doesn't allow any room for that sort of error. You've got to stay inside those Min and Max lines.

I just purchased a Gobe variable filter. It is supposed to have a 9 stop variation, however, my initial testing shows that it is only 5 stops. It wasn't very expensive ($30 CDN).
There are 18 settings between the min and max indicators. The first 7 or 8 setting seem to give the same exposure and then it gets progressively darker as it approaches the max setting. It appears that the change is not linear.
I agree with Donald that you cannot use outside the min and max setting.

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Originally Posted by Pixl8tor

I just purchased a Gobe variable filter. It is supposed to have a 9 stop variation, however, my initial testing shows that it is only 5 stops. It wasn't very expensive ($30 CDN).
There are 18 settings between the min and max indicators. The first 7 or 8 setting seem to give the same exposure and then it gets progressively darker as it approaches the max setting. It appears that the change is not linear.
I agree with Donald that you cannot use outside the min and max setting.

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The change should be sinusoidal. https://commons.wikimedia.org/wiki/File:Sinusoidal.png Changes in optical density are slower at min and max and more rapid in the middle.

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Originally Posted by paintingwithlight

The change should be sinusoidal. https://commons.wikimedia.org/wiki/File:Sinusoidal.png Changes in optical density are slower at min and max and more rapid in the middle.

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This makes sense with my results if I only look at half a wavelength.

Quote:

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Originally Posted by Pixl8tor

This makes sense with my results if I only look at half a wavelength.

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Think of one full rotation of the filter to be two wavelengths. Periodicity is when a waveform repeats itself. The periodicity of a 360 degree filter rotation is two.

Quote:

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Originally Posted by paintingwithlight

Think of one full rotation of the filter to be two wavelengths. Periodicity is when a waveform repeats itself. The periodicity of a 360 degree filter rotation is two.

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Thanks Michael. This explains exactly how my filter works.