A 
Japanese term used in 
photography, 
bokeh is the subjective 
aesthetic quality of out-of-focus areas of an image projected by a 
camera lens. For example, in some images the background may be deliberately caused to be out-of-focus to reduce distractions and to emphasize the primary subject. 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 are primarily determined by the circle of confusion. In out-of-focus areas, each point of light is instead 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.  
See also: depth of field.
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