Over the years, I've had many a supervisor ask me 'is that a blur or a defocus'? At the time, I thought a blur and defocus were the same thing. If I would have done any research into this, it would have been very obvious that there's a huge difference.
Notice how the last image looks. It look bad, it doesn't look like it came out of a camera. The highlights don't bloom out, they get smaller. The highlights don't get bright, they get darker. In the video tutorial below, I'll explain:
Here's a link to a frame of the CG that I used in the tutorial.