DOF depends on three things - aperture (F number), focal point distance (focus) and focal lenght (camera lens mm or FOV). In fact this is not entirely true, but let's not complicate things for ourselves.
I've attached very simple scene to test these things out. Open it, select camera object, launch Interactive Rendering. Now change F-stop value in CoronaCameraMod to something lower, like 8,0 or 4,0. Notice how DOF decreases (image becomes more blurry).
Next step, change Override Focus to different value (50, 100, 800), notice how shrp part of image changes its distance from camera.
Now scrub timeline. Camera focal length increases (20>24>28>35>50>85>135) and image becomes more and more blurry.
Play with settings and try to understand how each component affects DOF. You can even open online
DOF calculator in your browser, enter same focal length, aperture and focus point values as in your camera and watch how accurately Corona matches DOF rendering. Just remember to choose 35mm camera in online calculator (Canon 5D or Nikon Dx).
Have fun