Just as a complement to what Bruno rightfully said:

Raytraced DOF requires so much anti-aliasing that, in practice, I never use it (because it becomes impossibly slow). I always use Hybrid 2.5D. In my latest image, I used 3% camera blur and 6 passes. Using this many passes is very time consuming but, in my opinion, it's well worth the time.

An hint regarding camera target:
In practice, it's difficult to manipulate the camera target manually and still keep the POV you want. I do the following:
a) I create a very small cube (1mm) and place it in the place that I want in focus.
b) I make the cube invisible to render
c) with the camera selected, go to the object properties panel and click on "switch to target"
d) in the "focus on" field, select that mini-cube you created before.
e) you can now move the mini-cube to wherever you want and the camera target will follow, but you always keep the same POV. The area near the mini-cube will be in focus.

Hope that helps.