I tested the PolarPro Variable ND, 2- to 5-stop version, in the field. The sun moved in and out of the clouds, so I used both 2- and 4-stops on the filter. The objective was to maintain the aperture at F4.0 to minimize DOF while keeping the 180-degree rule:

I have just today sent back to Amazon two sets of Cokin Z154 ND8 filters. They slipped out of my 4x4 filterholder, since they aren't really 4" by 4" but 100mm x 100mm. The difference seems marginal but it was enough to make them unusable in my mattebox. I then ordered Tiffen 4x4s.

Edit: My filterholder is 103,8mm tall and those little "lips" were to short to grab on to the filter.

@Manu4Vendetta

Well, yes.

Check https://www.ephotozine.com/article/cokin-nuances-nd1024-10-stop-filter-review-27892

It is new premium Cokin filters, they finally promise good performance.

Time for some more ND filter tests! This time, 4x4" glass filters. I've owned and used Schneider 4x4 Tru-pol and grad ND filters for at least 7 years and have been perfectly happy with them. See the first photo. (All have a fixed daylight white balance and are ISO 200)

So when I needed a (non-grad) 4x4 ND1.2, I ordered the Schneider one from B&H, product number 68-041244. I was quite shocked to find it had a strong purple cast to the image, and it didn't look like IR contamination, which shouldn't be an issue anyway on my GH2 with that level of ND. See the second photo.

I got in touch with B&H, who told me that the colour cast was to be expected. So I contacted Schneider who were extremely helpful, and I replaced it with a 4x4 Platinum IRND1.2, product number 68-061244. This one looks beautiful. See the third photo.

I wanted to pick up an ND0.6 (AKA ND4) next, but I couldn't afford another platinum, and I was now really skeptical about the quality of the regular Schneider line, especially after reading B&H review comments like: "This, like all the Schneider filters I bought, shifted the colors so significantly to red/pink that I sent it back." http://www.bhphotovideo.com/c/product/251624-REG/Schneider_68040644_Neutral_Density_ND_0_6.html#costumerReview"

I decided to give the "Glass 4x4 ND Filters" available here in the deals section. I figured, being quite picky about filter quality, that I was risking $41, but that was better than risking $170. http://www.personal-view.com/deals/filters/other-filters-holders

The result? See the fourth photo. It was taken at a completely different time of day, and please excuse my thumb in the photo, but I'm seeing no loss of sharpness on a 16 megapixel still, and the colour is perfect, just like the $260 Schneider Platinum! Thank you, Mr. Kiselev!

(I just realized the first three photos had sharpening added, but the fourth didn't. So I re-uploaded the fourth with sharpening. I don't have permission to delete the third image, but if a mod could do it...)

@_gl My Fotga Slim Wide Band Fader ND polarizes just the same no matter which way it is facing.

As for the rings you are looking for, Fotodiox makes them, for one. http://www.fotodioxpro.com/catalogsearch/result/?q=filter+to+filter+reversing

Also, this company on eBay: http://stores.ebay.com/yuanchuangli/Double-Coupling-Ring-Adapter-/_i.html?_fsub=5380295016

@_gl I just saw your post from July 2011 (http://personal-view.com/talks/discussion/comment/5377#Comment_5377) talking about RealD 3D glasses. The unpleasant colour shifts you mentioned are due to the fact that you had two circular polarizers. This was actually an intended effect with the Cokin 173 Varicolor blue/yellow, and the Singh-Ray Gold-N-Blue Polarizer.

Note that RealD 3D glasses have the quarter-wave retarder in front, unlike the typical circular polarizer filter, so this may play in to your experiments.

.. thinking about it, a traditional fader design (front->linear,linear,retarder) is exactly the opposite of mine. So maybe they simply reversed it by mistake! Not unheard of for cheap generic stuff.

Which also means that (if you don't need phase AF), you could just reverse a standard fader to get the same effect! I've never tried to remove glass from a filter, maybe it can be done? Or can you get adapter rings that allow you to mount a filter backwards?

@_gl I was trying to keep things simple with "unpolarize" but yes, randomizing the polarization is a better description of a quarter-wave retarder. What I said may have made it seem like it reverses the effect of the polarization, which is not the case. To be technically accurate, a circular polarizer converts unpolarized light into linearly polarized light, and then into circularly polarized light.

Usually a circular polarizer has the quarter-wave retarder at the back, and I thought you were suggesting finding (or creating a stack of) a circular polarizer then a linear one, which would end up as linear polarizer, quarter-wave retarder, linear polarizer, which wouldn't give you a fader.

If what you've found is a specially-made fader that is quarter-wave retarder, linear polarizer, linear polarizer, then it should work as you described, but I've never seen one.

A fascinating discussion - usually I don't get much further than people thinking a circular polarizer means the filter is physically circular, rather than square!

I purchased a cheap ND32 filter from eBay, and the brand name wasn't listed. It was cheap, and it turned out to indeed be too good to be true.

What arrived was branded "SINGLEE" on the package, and the filter had only "ND32 72mm" printed on the ring.

The filter is not glass, it's plastic. It came pre-smudged, and was very difficult to clean, probably the worst thing about plastic filters.

It's supposed to be an ND32, but it's denser than a high-quality ND64 that I own. There is an unbelievably strong purple cast to the filter, and once that is corrected for, I was left with a very low-contrast image.

It's totally unusable.