From BlenderWiki
2.6 DOF setting.
First, click on your camera, select under the Camera Icon, the check box Limits.
Inside the camera block, there is a blank box with "Depth of Field" typed above it. Click the box and choose the item you would like the focus to stay on. - If you want to vary the focus with an invisible control, add an empty and select that with the empty box drop down choice now available, then animate the Empty's location over a 100 frame time... It will focus like a movie.
Next, change main 3D view to Nodes.
Once you see the Nodes icons at the bottom of the screen, click the little "looks like" double paper icon, then "Use Nodes check box". THIS NEEDS TO BE DONE BEFORE NEXT STEP!
Using lower menu, click Add - Filter - Defocus
Wire the new Defocus node in between the two existing panels and include the z on the first half.
Change your center panel values to FStop 6 Max Blur 7 Threshold 0 to start.
Your good, try a Render - I wish I could have put screen shots on this.
