I'm new to Corona (as of Oct 2016), and I currently have some free time to explore and enjoy this wonderful new render engine I'm am really warming to (I've been using VRay for years). 3DS Max 2017, Corona 1.5.
For me though, DOF in Corona has taken a little head-scratching to understand and especially to ask myself why is it in several places ?
The rule I seem to have told myself thus far regarding Corona is that under Camera Tab of Render Setup - the DOF Enable doesn't seem to affect anything..... (right or wrong ? and if wrong, please can you explain).
So, with the vanilla version of 3ds Physical Camera ALONE, you can play with DOF as you wish.
But when you apply a CoronaCameraMod modifier you are able to OVERRIDE that Physical camera's settings - I guess so that you can have different Physical Cameras within your scene, each with different CoronaCameraModifiers (and DOF's).
PS - the Docs seem to be missing VITAL piece of info (on hover) for this very area in question as of 14/10/16 here -
https://corona-renderer.com/doc/