What would cause DOF to fail?

[Boofredlay]

Thanks again guys for the insights. Fletch, I can send it to you tonight.

[Fletch]

Well, Miguel of course hit the nail on the head.
Your scene is 400m
I rescaled to 4m long,
positioned my view,
set my target to be on distant object
changed cam to 2.8 F number
and here you go... Easy Low... stopped when obvious it works.

I would post-pro DOF with Depth rendering, but you use unbiased and are not in a hurry, so...

[Boofredlay]

Thanks.