http://richardrosenman.com/shop/dof-pro/
This is a proprietary plugin for replicating depth of field in Photoshop. I have some qualms with the plugin, but so long as you're happy with both a destructive and a less-than-floating-point workflow, this tool gets good results for a fraction of the expense of rendering in depth of field with a raytracer.
What I'm putting this up for though is Rosenman's highly detailed breakdown of the circle of confusion phenomenon that produces what we commonly refer to as 'depth of field'.
For anyone wanting elucidation on the intricacies of the physics involved that produce this effect, this website serves as a good primer.
Enjoy.