Of course many of us want to always nail a shot in camera, but sometimes things happen. Maybe it looked great on your camera's LCD, but entirely different once you got it on your computer. Maybe your sensor is too small and shallow DOF is hard to nail. It's for these cases that Aaron Nace at PHLEARN does tutorials like this one, where he shows how to adjust depth of field in post.
"Rather than using our beloved gaussian blur, we’ll be using lens blur which does a much better job of imitating the blur produced by an actual lens. By selecting the channel with our depth map as the depth map source, the blur is applied more to the parts of the image further fromt the camera and less to the parts of the image closest to the camera. Just like a real shallow depth of field!"
Below you can see the before on the right, and the adjusted depth of field on the left.