Another way of having good control over a rendererd DOF-Effect is using the connection manager, Camera and Aim and the Distance Measure Tool.

I've created a small tutorial for you..no sound, just movie. Techsmith-Codec required...

Techsmith Codec

Depth of Field Tutorial

You can now adjust the effect, by simpy using the F-Stop slider, to adjust how big the sharp area should be. (hopefully it works )

Hope it's made clearly...and if you have any further questions...I'm here ! ;o)