The Nuke node “Defocus” is a great way to very quickly get good defocused results. However, when used on non-linear content it doesn’t work as originally intended.
This tutorial covers the workflow to successfully defocus non-linear content using the linear conversion node.
Nuke is a great tool from The Foundry, who’ve also created Mari, Modo (from Luxology) and Hiero, among others. Be sure to check them out!