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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.
The characteristics of a lens that cause it to produce pleasing bokeh are
not fully understood. Generally lenses with more blades in the diaphragm
tend to produce more pleasing bokeh, however this isn't always the case.
Zoom lenses often have poor bokeh but again, this isn't always the case.
Pleasing bokeh is often very important for a portrait lens (medium
telephoto) because the photographer very often selects a shallow depth of
field to make the subject stand out.
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