A little more info... I tried long exposures of 30-76 sec indoors without filter and found no artifact. I repeated this experiment outdoors and artifact is much less noticeable now. The light is much flatter now. I'm wondering if there is some kind of reflection artifact that is noticeable on an Fx and not a DX ? related to differences in distance of lens to sensor. I'm thinking my camera is probably okay but there is a problem with this filter only on this camera. What do you think?
"Cameras and lenses are simply tools to place our unique vision on film. Concentrate on equipment and you'll take technically good photographs. Concentrate on seeing the light's magic colors and your images will stir the soul." Jack Dykinga