Q
Why does vignetting occur?
Answer Question
A
Vignetting is a precursor to the appearance of a black cross in an adjustable ND filter. All adjustable ND filters will have this problem. The larger the focal length used, the smaller the impact of the cross shadow. At large ND settings, there will be obvious cross shadows and vignetting. Continue to rotate and the shadow will disappear naturally. This is a physical phenomenon, not a product problem.