I am not sure if you tried real world camera lens settings for the depth of field, I never got them to work as (I) expected.
So, first I put quite a large value into the "Aperture diameter in mm" field of the camera. In the example you can see it says
"f/0.3 with 28mm focal length". I have no idea if these values relate to something you would have on a real camera lens.
Anyways, then I try to find a "Focus distance" that somehow gives a visible depth of field in the scene.
To use helper objects in the scene depth (like a few cylinders) gives you a much easier hint of distances
then trying to figure it out in the terrain alone. There is also the measuring tool to help you along.
It is trial and error mostly for me. Important is the 3D viewer to guide you along.
The light brown colour in the viewport indicates where depth of field starts.
The uncoloured part of the viewport is in sharp focus (more or less).
Have a look at the project I provided and play around from there.
I think, there is a good article in the wiki too.
Hope that helps.