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I've been using a Singh-Ray vari-ND with my Panasonic GH1, and I'm seeing obvious dark spots in the image. On a lower setting the left and right side of the image have dark lines and when I go to a maximum setting the dark sides cave into the middle and forming a circles and it looks really bad. I've been using a 20mm lumix 1.7 lens, and I read that the vari-ND shouldn't be used with lenses wider than 35mm in full-frame terms (which mine would be a 40mm after the micro 4/3 conversion) due to the polarized filter. Could this be why? Or is my filter defective?

medium setting on filter low setting on filter high setting on filter (my hand in slightly in the pic too) Note that my hand is slightly in this last image, and is the cause of the darkened upper-right corner.

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    Can you post an example pic? – rfusca Jan 7 '12 at 23:39
  • Is your filter mounted directly to the lens, or with any kind of stepup/down adapter? – rfusca Jan 8 '12 at 0:16
  • Yes the vari-ND is 77mm and my lens is 46mm so I have 2 step down rings 77-52mm and 52-46mm. Plus a hoya 77mm UV filter on the front. – trying_hal9000 Jan 8 '12 at 0:21
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    I would suggest removing the third image, it is somewhat confusing to include an image that has your hand in it in a question that you are asking about dark spots in the picture in. – dpollitt Jan 8 '12 at 5:05
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The third image looks like vignetting due to the thick Sing-Ray filter and your step down rings. But strange it's only in one corner - you don't have a hood attached do you? You should be able to see this vignetting through the viewfinder.

Even a normal polarising filter is going to result in uneven skies with wide angle lenses, with the sides often being much darker, due to the wide field of view (light coming from vastly different angles through the filter). With variable ND filters, this effect is multiplied.

The solution is using a longer focal length, if possible, cropping out the sides, or reducing the filter density.

The number of stops you can reduce with these filters goes down as the lens focal length goes down. You might be able to reduce 8 stops at 135mm, but only 5 stops at 20mm.

Your 20mm (effectively 40mm) is close to the 35mm limit they mention. It's a gradual thing, so if it's not recommended at 35mm, it's probably not ideal at 40-45mm and you can expect some vignetting or strange colour casts at the edges, and these will move more to the centre of your images the wider you go.

From their website:

NOTE: Due to the nature of the Vari-ND Filter, its "profile thickness" is significantly greater than most filters (14.2mm for standard mount, 10.5mm for thin mount). The profile thickness of either version may cause vignetting when used with wide-angle lenses, especially on cameras with full-frame sensors. Due to the vast number of combinations of cameras, lenses, and other factors, we can NOT predict under what circumstances vignetting will occur, and to what degree. We suggest simply adjusting your focal length, position, and/or composition to remove the vignetting -- use your digital camera's display for reference.

NOTE 2: The design of the Vari-ND Filter may introduce irregularities when used with very wide angle lenses, especially on full-frame cameras. Adjust your focal length and reduce the filter density setting until the irregularities disappear

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    that's actually my hand in the last picture as I'm adjusting the nd filter. Would you be able to tell me if this same problem would occur if I was stacking individual ND filters or if its specific to to the variable ND which are polarized? – trying_hal9000 Jan 8 '12 at 0:54
  • You can get strange colour casts if you stack ND filters (or combinations of ND and polarising filters). I don't think you would get banding at the sides though with just ordinary stacked ND filters, as I assume that is a polarising effect. Of course if you stack too many things you'll end up with a vignetting problem if the filters/holders completely block your wide angle view. – MikeW♦ Jan 8 '12 at 1:01
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I imagine you're getting the vignetting because of the large amount of things on the end of your lens - 2 filters and 2 step down rings.

At least try removing the UV filter, but you might have to resolve to cropping or editing it out digitally. Try holding the ND filter all by itself in front of the lens, right up against the front element where it would be if screwed in - and I bet you don't get the vignetting.

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