You probably going to have to play around with Corona's in render DoF (depth of field). You can enable it in the Corona render settings page and play around with the focus distance and f-stop etc.
But imo the best way to do it, is to add a Z-depth pass and have After Effects render the DoF. And in AE you can set the amount and quality of the blur too.
Here is an example of a z-depth + After effects I used.