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ACNY, the phenomenon he is seeing is different from the familiar problem of uneven density in sky shots with a single polarizer. This pattern affects the entire field, whether the sky is involved or not, and relates to interference between the two polarization angles, and not a differential the polarization of the light itself. It relates to wide angle lenses only because the shorter focal length "sees" more of the filter, and thus more of the pattern, and the difference between lighter and darker parts is more evident; it is not caused by using a wide angle lens. It doesn't seem to be a problem with variable ND at lower densities, so the variability feature is still useful so long as you don't try to go too far. And I agree that the surest way to avoid it is to use a conventional ND, or different NDs in various combinations, to get the degree of density needed.