Tried some Terragen DOF, but had to alter the camera settings. The original render uses an 80 mm lens setting and it absolutely would not render out with DOF turned on. Even a tiny render at .4 detail and 4 AA 600x300 took over an hour to finish just one row of buckets and then the result was just black with no details, although the prepass shows normal details. Only the most mild (small aperature setting) DOF would render properly in five or six minutes. Not sure if the problem is the overall darkness or the 80mm lens or both.
This render used a 45mm camera setting and full sunlight instead of a spotlight.