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Some lenses are thought to produce more pleasing out-of-focus areas that enhance the over-all quality of the image. Bokeh is a quite subjective quality that is difficult to quantify and is sometimes debated.
Bokeh characteristics may be quantified by the circle of confusion. In out-of-focus areas, each point of light becomes a disc. In some lenses, that disc is uniformly illuminated, for others it is brighter near the edge, and for others it is brighter near the center. Some lenses show one kind of disc for out-of-focus points closer to the camera, and a different kind for points far from the camera.
The shape of the aperture is known to have a great influence on bokeh. Mirror lenses produce a "doughnut" bokeh that is generally considered as unpleasant. Some lenses will show a many-sided shape rather than a circle, depending on the blades in their diaphragm. Generally lenses with more blades in the diaphragm tend to produce more "round" bokeh.
The other characteristics of a lens that cause it to produce pleasing bokeh are complicated. Most photographers do not understand how the lens design influences bokeh, they merely note whether a lens produces results that please them.
Bokeh is especially important for large aperture macro lenses and long telephoto lenses becauses they're typically used with a narrow depth of field.
Bokeh also important for "portrait lenses" (typically medium telephoto - 85-150 mm on 35 mm format) because the photographer would typically select a shallow depth of field (wide aperture) to have an out of focus background, to make the subject stand out.
See also: depth of field, aberrations of lens.
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