Depth of Field

What is Depth of Field (DOF)? In my opinion, camera only focuses within a specific range. See this picture. Between these three balls, camera focus in most left ball and other ball seen blurry.
How to create this effect in Autodesk Maya?

Create object like picture bellow.

Select camera1 and check Depth of field. Like my opinion about DOF, this effect depent on range between camera and object. How I know range between camera and object?

To know the range between camera and object, you must activates object details (see picture below).

After object details active, you will see this text on scene.

Now you must select the camera (this is important!!!).

After you select the camera, now select the object. You will see the distance on scene (in my scene distance between camera 1 and ball 1 is 4.3).
Distance between camera 1 and ball 2 is 12.3.
Distance between camera 1 and ball 3 is 18.3.

I want focus on ball 1, so I must set Focus Distance 4.3 and for F Stop I always do trail and error in this case I set F Stop to 7.

Focus Distance 4.3, F Stop 7.

Focus Distance 12.3, F Stop 4.

Focus Distance 18.3, F Stop 4.