Bokeh (derived from Japanese boke ぼけ, a noun form of bokeru ぼける, "become blurred or fuzzy") is a photographic term referring to the appearance of out-of-focus areas in an image produced by a camera lens using a shallow depth of field. Different lens bokeh produces different aesthetic qualities in out-of-focus backgrounds, which are often used to reduce distractions and emphasize the primary subject.
Mike Johnston, former editor of Photo Techniques magazine, claims to have coined the bokeh spelling to suggest the correct pronunciation to English speakers, replacing the previous spelling boke that derived directly from the Japanese word for "fuzzy" and had been in use at least since 1996. It can be or (boke-aay or boke-uh).
The term bokeh has appeared in photography books at least since 2000.
Although difficult to quantify, some lenses enhance overall image quality by producing more subjectively pleasing out-of-focus areas, referred to as bokeh. Bokeh is especially important for large-aperture lenses, macro lenses, and long telephoto lenses because they are typically used with a shallow depth of field. Bokeh is also important for medium telephoto "portrait lenses" (typically 85–150 mm on 35-mm format) because the photographer would typically select a shallow depth of field (wide aperture) to achieve an out-of-focus background and make the subject stand out
Bokeh characteristics may be quantified by examining the image's circle of confusion. In out-of-focus areas, each point of light becomes a disc. Depending how a lens is corrected for spherical aberration, the disc may be uniformly illuminated, brighter near the edge, or brighter near the center. Lenses that are poorly corrected for spherical aberration will show one kind of disc for out-of-focus points in front of the plane of focus, and a different kind for points behind. This may actually be desirable, as blur circles that are dimmer near the edges produce less-defined shapes which blend smoothly with the surrounding image. Lens manufacturers including Nikon, Canon, and Minolta make lenses designed with specific controls to change the rendering of the out-of-focus areas.
The shape of the aperture has a great influence on the subjective quality of bokeh. When a lens is stopped down to something other than its maximum aperture size (minimum f-number), out-of-focus points are blurred into the polygonal shape of the aperture rather than perfect circles. This is most apparent when a lens produces undesirable, hard-edged bokeh, therefore some lenses have aperture blades with curved edges to make the aperture more closely approximate a circle rather than a polygon. Lens designers can also increase the number of blades to achieve the same effect. Traditional "Portrait" lenses, such as the "fast" 85mm focal length models for 35mm cameras often feature almost circular aperture diaphragms, as is the case with Canon's EF 85mm f/1.2L II lens and Nikon's 85mm f/1.4D, and are generally considered exceptional performers.
Leica (formerly Leitz, "Leica" only referring to the camera body until more recently) lenses, especially vintage ones, are often claimed to excel in this respect, although Leica photographers have tended to make more use of maximum aperture due to the lenses' ability to maintain good sharpness at wide openings and the suitability of the Leica camera system for available-light theatre work and reportage. Consequently, more evidence is needed to determine whether Leica's lens designers deliberately set out to produce pleasing bokeh.