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Canon EOS 40D Guide to Digital Photography by David D. Busch

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Telephotos and Bokeh

Bokeh describes the aesthetic qualities of the out-of-focus parts of an image and whether out-of-focus points of light—circles of confusion—are rendered as distracting fuzzy discs or smoothly fade into the background. Boke is a Japanese word for “blur,” and the h was added to keep English speakers from rendering it monosyllabically to rhyme with broke. Although bokeh is visible in blurry portions of any image, it’s of particular concern with telephoto lenses, which, thanks to the magic of reduced depth-of-field, produce more obviously out-of-focus areas.

Bokeh can vary from lens to lens, or even within a given lens depending on the f/stop in use. Bokeh becomes objectionable when the circles of confusion are evenly illuminated, ...

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