Aperture diffraction allows the simulation of light diffraction through the camera aperture. Such diffraction creates a distinct "bloom" or glare effect around bright light sources in the image. The shape of the glare effect is determined by the shape of the aperture.
You can enable or disable aperture diffraction via the "Enable aperture diffraction" checkbox in the imaging section of the render settings view.
Post-process aperture diffraction dramatically increases the amount of memory (RAM) used, and the time taken to update the displayed image.
The following images illustrate the effect of aperture diffraction with various aperture shapes:
A simple render of the sun, without aperture diffraction enabled.
Aperture diffraction with a circular aperture.
Aperture diffraction with a 6-blade generated aperture.
Aperture diffraction with an 8-blade generated aperture.