The fstop feature is being used incorrectly.  You need to establish a distance for your model, select an appropriate fstop, and set depth of field in your render settings.  Pixel sampling required for this feature: 10 to 15.  Dr. Geep has more information how to set this up.

www.drgeep.com

BTW, it is not necessary to maximize your render settings.  Otherwise, you will find your render times increasing significantly to the point of crashing.  Also, you must check your materials for bump and displacement mapping.  If such mapping does not exist, activating displacement in your render will do nothing.