As most of you know I shoot landscapes and often use ND filters for long exposures. One such example is posted below. I've been having a problem with these long exposures now for a number on months and cant solve the issue which I've described below. I'm looking for wiser minds than mind to help solve this before I start my summer travels to collect new images. If you help solve the problem, I just might be able to capture some descent images to share with you. :-) I leave your future viewing pleasure in your hands. All suggestions (solutions) are welcome.

I shoot a Nikon D800 and D810 with a 24-70mm lens. I use filters for long exposures. I have two B&W ND filters, one a 6 stop and one a 10 stop. I have no problems using the 6 stop the issue is with the 10 stop. I did not notice the problem at first but now is it constant. I am attaching a photo shot in Maine I have marked with an arrow and underlined the problem. As you can see there is a magenta line running from left to right across the image. It is reduced here as much as I can in post, but it is still there. For long exposures using the ND filter I usually shoot at my lowest ISO (64 or 100 depending on the camera) with an Fstop from 16 to 22. The ND filter is circular. The magenta line always appears in roughly this position that is in the lower third of the horizontal (or vertical image). It does not matter if I rotate the filter by say 90 degrees, the line still appears in the same spot. Exposure times vary from 20 seconds to several minutes with the same results.

I've cleaned the filter, and there are no visible markings on the glass. Using or not using a lens shade makes no difference. I really can't sell any of the images with the magenta ban and have about reached the point where I just don't use the filter.

While my filter is from B&W, I recently hear from a friend that he is having a similar problem with his 10 stop purchased from Breakthrough Technologies. I've seen images online using a 10 stop with out the band. What am I doing wrong?

Thanks for any suggestions.