To achieve depth of field, you first need to choose a target object from the drop down list; this done in the object properties panel when the camera is selected. once this is done, click the caùera icon to go back to the camera settings.Check your views, you'll see there is a checkbox at the end of the camera's filed of view ( focus), change thisvalue until the checkbox reaches your trget object. Next, add some blur to your camera,  the more blur you add the more blur around your target object. You will see 2 vertical lines inside the camera's field of view. What is inside the 2 bars is in focus, by bumping up the blur %, you shrink the focus zone around you target object.