From Wiki: http://en.wikipedia.org/wiki/Bokeh

(emphasis added by me)

In photography, bokeh (Japanese pronunciation: [boke]) is the blur, or the aesthetic quality of the blur, in out-of-focus areas of an image, or "the way the lens renders out-of-focus points of light." Differences in lens aberrations and aperture shape cause some lens designs to blur the image in a way that is pleasing to the eye, while others produce blurring that is unpleasant or distracting**—"good" and "bad" bokeh**, respectively. Bokeh occurs for parts of the scene that lie outside the depth of field. Photographers sometimes deliberately use a shallow focus technique to create images with prominent out-of-focus regions.

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Bokeh is also important for medium telephoto "portrait lenses" (typically 85–150 mm on 35 mm format) because in portraiture photography, the photographer typically seeks to obtain a shallow depth of field to achieve an out-of-focus background and make the subject stand out.

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