Bokeh

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This article is about the photographic usage of the term "Bokeh". For other uses of the term, please refer to Bokeh (disambiguation)

Bokeh on a photo shot with a 85mm lens and 70mm aperture, which corresponds to f/1.2

Bokeh on a photo shot with a 200mm lens and 71.4mm aperture, which corresponds to f/2.8

Bokeh (from the Japanese boke ぼけ, "blur"^[1]) is a photographic term describing the subjective aesthetic qualities of out-of-focus areas in an image produced by a camera lens. For example, causing an out-of-focus background image may reduce distractions and emphasize the primary subject.

Although difficult to quantify, and hence open to debate, some lenses are believed to enhance overall image quality by producing more subjectively pleasing out-of-focus areas (bokeh). Bokeh is especially important for large-aperture lenses, macro lenses, and long telephoto lenses because they are typically used with a narrow depth of field. Bokeh is also important for "portrait lenses" (typically medium telephoto — 85–150 mm on 35-mm format) because the photographer would typically select a shallow depth of field (wide aperture) to have 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, that 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. Nikon makes lenses with a Defocus Control function in popular portrait lengths (Nikkor 105mm f/2 and Nikkor 135mm f/2) which allow the user to increase the amount of uncorrected spherical aberration to change the rendering of the out-of-focus areas.

The shape of the aperture is commonly believed to have a great influence on the subjective quality of bokeh. When a lens is stopped down to something other than its maximum aperture, out-of-focus points are blurred into the polygonal shape of the aperture rather than perfect circles. However, this is only apparent when a lens produces undesirable, hard-edged bokeh. 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.

Bokeh can be approximated by convolving an image with a circular box blur kernel. In general, a particular look can be achieved by convolving with a kernel corresponding to the image of an out-of-focus point source taken with a real camera.

No bokeh

Faux bokeh

The other characteristics of a lens that cause it to produce pleasing bokeh are complicated. Some lens designs are known to produce a certain kind of bokeh, but most photographers do not fully understand how the lens design influences bokeh; they merely note whether a lens produces results that please them. For example, mirror lenses produce a "doughnut" bokeh that is generally considered unpleasant.

[edit] Notes

^ Mike Johnston, former editor of Photo Techniques magazine, claims to have coined the "bokeh" spelling to suggest the correct pronunciation to English speakers; see his Bokeh in Pictures article referenced below.