I’ve found in using the HPX-370, that when you close the lens down to a small f-stop, it will affect the sharpness of the image. I never shoot with a setting smaller than f-8 and find the sweet spot to be around f-4 to f-5.6. That’s why the image gets better when you dial in some extra ND. Don’t know if it’s a diffraction problem from the iris blades, the small sensor size or probably a combination of both. Keep the iris around f-4 to f-5.6 and the camera should give you a sharp image.
E=MC2+/-2db