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, making them stand out as distinct discs, or, worse, when these circles are darker in the center, producing an ugly “doughnut” effect. A lens defect called spherical aberration may produce out-of-focus discs that are brighter on the edges and darker in the center, because the lens doesn’t focus light passing through the edges of the lens exactly as it does light going through the center. (Mirror or catadioptric lenses also produce this effect.)