The Depth of field section trades the pinhole camera for a real lens. Only what sits at the focus distance comes out sharp; everything else spreads into a circle that grows wider the farther it is from the focus plane.

Controls

Enable depth of field: Turns the lens on. With it off, everything is sharp from the foreground to infinity.
Focus distance: The distance in meters between the camera and the plane that stays sharp. In the images above the focus is on the front sphere.
Aperture (f-stop): The f-stop, from 0.5 to 32. A low number is a wide lens, with little in focus; a high number closes the lens and brings the whole image back.
Bokeh blades: The number of diaphragm blades, from 0 to 12. From 3 up they shape the blur of a point of light: with 6 blades, a small out of focus highlight becomes a hexagon. Below that the aperture is round.

How to use it

Measure the distance in the project itself: put the camera where it is going to sit and use the distance to the main object as a starting point, adjusting until its edge is sharp.

The blur comes from the lens itself, not from a smudge applied at the end, so each sample sees the image through a different point of the lens. With few samples the blur looks dirty: when you turn depth of field on, raise Antialiasing samples on the quality page.