Two of my friends were complaining about inability to get sharp focus on the same 17-35mm Nikkor zoom, and wondering if it was faulty glass. As we dug through variables we discovered the issue was more about being set to continuous AF when they should have been on single servo, and the fact that the camera acquired focus more quickly than they were used to and was often catching detritus in the water rather than the intended subject. But, while we were at it, we did some pool tests to determine which port extension was best with Seacam supedome, test PVL (port extension, in mm) 20, 25, 30, 35, 40, 45, 50, 55, and 60. We also tested for sideways motion and forward motion, and how efficiently it acquired focus in single servo.
Both motion tests looked great, as they dealt only with performance in the center of the frame.
Scenarios looked like this: