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Bokeh and lens design
The depth of field is the region where the size of the circle of confusion is less than the resolution of the human eye.
Though difficult to quantify, some lenses have subjectively more pleasing out-of-focus areas. "Good" bokeh is especially important for macro lenses and long telephoto lenses, because they are typically used in situations that produce shallow depth of field. Good bokeh is also important for medium telephoto lenses (typically 85-150 mm on 35 mm format). When used in portrait photography (for their "natural" perspective), the photographer usually wants a shallow depth of field, so that the subject stands out sharply against a blurred background.
Bokeh characteristics may be quantified by examining the image's circle of confusion. In out-of-focus areas, each point of light becomes an image of the aperture, generally a more or less round disc. Depending on how a lens is corrected for spherical aberration, the disc may be uniformly illuminated, brighter near the edge, or brighter near the center. A well-known lens that exhibited the latter "soap-bubble" characteristic was that produced by Hugo Meyer & Co., more recently revived by Meyer Optik Görlitz.[11]
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.[12] 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. The shape of the aperture has an influence on the subjective quality of bokeh as well. For conventional lens designs (with bladed apertures), when a lens is stopped down smaller than its maximum aperture size (minimum f-number), out-of-focus points are blurred into the polygonal shape formed by the aperture blades. This is most apparent when a lens produces hard-edged bokeh. For this reason, some lenses have many aperture blades and/or blades with curved edges to make the aperture more closely approximate a circle rather than a polygon. Minolta has been on the forefront of promoting and introducing lenses with near-ideal circular apertures since 1987, but most other manufacturers now offer lenses with shape-optimized diaphragms, at least for the domain of portraiture photography. In contrast, a catadioptric telephoto lens renders bokehs resembling doughnuts, because its secondary mirror blocks the central part of the aperture opening. Recently, photographers have exploited the shape of the bokeh by creating a simple mask out of card with shapes such as hearts or stars, that the photographer wishes the bokeh to be, and placing it over the lens.[13]
Lenses with 11, 12, or 15 blade iris diaphragms are often claimed to excel in bokeh quality. Because of this, the lenses do not need to reach wide apertures to get better circles (instead of polygons). In the past, wide aperture lenses (f/2, f/2.8) were very expensive, due to the complex mathematical design and manufacturing know-how required, at a time when all computations and glass making were done by hand. Leica could reach a good bokeh at f/4.5. Today it is much easier to make an f/1.8 lens, and a 9-bladed lens at f/1.8 is enough for an 85mm lens to achieve great bokeh.
Some lens manufacturers including Nikon,[14] Minolta, and Sony make lenses designed with specific controls to change the rendering of the out-of-focus areas.
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14 окт 2024
