I'm using a Nikon PB-5 bellows with a Micro Nikkor 55mm @ f/11. Exposure is with flash. Nikon D600. What I thought was sensor dust turns out to be something utterly baffling. Is it possible that I'm focusing the internal lens dust? The camera sensor checked out O.K. It had a few soft spots, but nothing like the hundred or more hard black dust spots that clog both ends of the frame length wise. I'm using a focus staking technique and noticed that the out of focus areas have sharper dust spots, and they are many. What to do? I'm truly surprised to discover this problem, and it occurs with both the d7000 and the d600. I don't think it's the camera.