I just got a new Hoya Pro1D 64x ND filter so I decided to take it out on a test run. I picked the Hoya over B+W for the supposed lack of color casting in the image. True enough, there was no apparent color cast in my images.
When I got back home, I was just aiming the camera at random targets and shooting. I noticed then that the images had a ripple-like pattern exactly at the middle of the frame (see attached picture). I swapped out the ND filter for my B+W UV one and the pattern was gone. Going back to the ND resulted to the pattern again. Strangely enough, going to the long end of my lens resulted to a larger ripple (much like a zoomed version). Inspection of the filter showed no dirt except some strange residue on it that is only visible when wet (see second picture).
After rigorously cleaning the filter (enough to maybe even leave some scratches
), I put it on the camera and the pattern was there. I cleaned it again and the pattern was completely gone this time. I really have no idea what caused this. The residue wasn't at the center so a perfectly centered ripple seems strange.
Anyone have any ideas?