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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. |
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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.
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