First of all, pmcf1981 nice looking renders:) Was there a lot of cleanup to do in the DOF noise or did it handle well? Being a Corona beginner myself, I'll have to check some settings to sample DOF better.
Second: Doing DOF in a render pass is never a good way to go, so if you have time to wait, as renders usually take a bit longer with DOF, you should wait :)
The problem is, when taking pics/render with DOF, the light is "curved" around objects, or better - because of the size of your aperture (fstop) there are rays shooting out of the camera in a larger area... so it captures more information around the back. The render engine knows that, while if you do dof in post, there isn't any information in the image about the stuff behind the blurred object, so it only blurs where the frontal object would be, without the info behind it... I guess kinda like a pinhole camera, where there's basically infinite depth of field. I've explained the way I understand it... I remember reading about it somewhere but have no idea where...
