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 mono-syllabically 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.)
Other kinds of spherical aberration generate circles of confusion that are brightest in the center and fade out at the edges, producing a smooth blending effect, as you can see at right in Figure 6.11. Ironically, when no spherical aberration is present at all, the discs are a uniform shade, which, while better than the doughnut effect, is not as pleasing as the bright center/dark edge rendition. The shape of the disc also comes into play, with round smooth circles considered the best, and nonagonal or some other polygon (determined by the shape of the lens diaphragm) considered less desirable.
If you plan to use selective focus a lot, you should investigate the bokeh characteristics of a particular lens before you buy. Canon user groups and forums will usually be full of comments and questions about bokeh, so the research is fairly easy.
Add-ons and Special Features
Once you've purchased your telephoto lens, you'll want to think about some appropriate accessories for it. There are some handy add-ons available that can be valuable. Here are a couple of them to think about.
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