Why am I getting dark spots with my Singh-Ray 77mm vari-ND filter?

Question

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?

Note that my hand is slightly in this last image, and is the cause of the darkened upper-right corner.

Answer

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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