Last time I checked, blender could only render depth of field effects in a post process pass, using the "Defocus Node", which seemed to get it quite wrong.
Is there now a way to include rays across a lens during tracing and produce a more realistic depth of field?
With the Cycles Renderer, DoF is supported natively. Select the camera, switch to the Camera properties panel, and set up DoF.
Also you must use a real size object to obtain realistic result (to get a reliable f/stop response, like real world lenses). Small objects close up (at a short focal length) make a great bokeh effect due to a shallow DoF. Larger, distant objects like a building create poor bokeh (deep DoF). Also play with some camera presets, like sensor size and focal length.
