Understanding the Bokeh of a Digital Camera Lens
The quality of the out-of-focus areas of an image that a lens produces (its bokeh) has become a digital-photography obsession. Some lenses are prized for their wonderful bokeh, whereas others are reviled because their bokeh stinks.
Bokeh describes the points of light in the background that become fuzzy disks when rendered out-of-focus by inadequate depth-of-field. Various lenses render these disks in different ways:
Uniformly illuminated disks, like in this figure.
Disks that are dark in the center and lighter at the edges, which stand out like a sore thumb or, actually, a frosted doughnut.These light disks look like frosted doughnuts.
A nicely illuminated center that fades to a darker edge makes the disks tend to blend into each other.
Lenses with the very best bokeh characteristics display almost no disk shapes at all, as shown in this figure.This lens created a background with almost no disks visible.