I prsume DOF is depth of field?

Try using a small aperature, F16 or so. I also try and angle the camera, so that the sensor is more parallel to the subject. There is a good article about it here:-

http://www.cs.mtu.edu/~shene/DigiCam...-of-field.html

Can't tell what is the problem with noise/artifacts without seeing a 100% photo. I stop the lens down a bit more and only use the polarising filter when it reduces reflections and improves the photo, most of the time it isn't necessary and it might be causing your DOF problem, as you will be using a larger aperature with it on.

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