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Bokeh, created by manipulating the aperture

Bokeh is a term used to describe the quality of blurring in out-of-focus areas of an image. It is related to the camera's aperture and the quality of the lens. A point in an image out of focus is no longer rendered as a point, but a blurred disk. Depending on how the lens has been corrected for spherical aberration (imperfections caused by light rays striking the edge of a mirror), the disc may be uniformly illuminated (“constant” bokeh), brighter at the edge (“bad” bokeh), or brighter near the middle (“good” bokeh).

Poorly corrected lenses will show one type of disc in front of the focal distance and another type behind, which may be desirable, as these circles with a dimmer edge can produce smoother less-defined shapes in the surrounding image. When the aperture is stopped down from its maximum size the out-of-focus points are blurred into the polygonal shape of the aperture, instead of perfect circles. Some modern-day cameras have curved edges on the aperture blades, or a greater number of blades to make the bokeh closely match a circle as much as possible.

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