Here's an obscure one for you...
If you have two cameras in the scene with different names, but their targets have the same name (this can happen easily when you merge in the "same" camera from another file and auto-rename it - it renames the cam but not the target, thanks Autodesk...) no matter what you do the view will always render with some kind of DOF softness, even if you have Force DOF Off ticked in the camera mod.
Can't share the scene, sorry.