Just a quick comparison between rendering the quick way (A) and the sloooooooooow way (B). Guess which method I always use?
A: In vray's color mapping section, Sub-pixel mapping is ticked, as is Clamp output (set to a value of 1.0).
B: Both Sub-pixel mapping and Clamp output unticked.
Even if you aren't trying to get this particular lens effect, I still recommend rendering unclamped for trees (and many other situations), it just looks so much more realistic. When I get a chance, I'll render the same scene without DOF to show you the difference.
C: clamped, and sub-pixel mapping ON D: unclamped, sub-pixel mapping OFF
Even without DOF, I think it trees still look much more realistic when rendered unclamped.
* out of focus