Let's take a look at the ZDefocus node. This is the node you want to use with elements that come with a depth pass, like CG renders. It's gives you a lot on control to make beautiful and realistic out-of-focus images.
Check out the written version here: jonathanjobin.com/youtube-videos/zdefocus
Previous video about Blur & Defocus here: https://youtu.be/GpRRAvscErw
If you have any questions, ask me in the comments. I'll be happy to help. If you enjoyed this video like it and subscribe to my channel for more.
