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(詳細はaperture diameter, or minimum f-number, of a photographic lens. A lens with a larger maximum aperture (that is, a smaller minimum f-number) is called a "fast lens" because it delivers more light intensity (illuminance) to the focal plane, achieving the same exposure with a faster shutter speed. It also needs to reduce interference from extraneous (light) noise i.e. lower the noise floor. A smaller maximum aperture (larger minimum f-number) is "slow" because it delivers less light intensity i.e. has a higher noise floor and requires a slower shutter speed. A lens may be referred to as "fast" or "slow" depending on its maximum aperture compared to other lenses of similar focal length designed for a similar film format. Lens speed given by the minimum f-number, or alternatively maximum aperture diameter or maximum numerical aperture, is a useful quantitative way to compare similar lenses. Lens speed is important in taking pictures in dim light, or with long telephoto lenses. For controlling depth of field, especially in portrait photography,〔Waldren, Margaret (and others) ''Advanced Digital Photography'' 2004 Media Publishing〕 lens speed is a key variable in combination with other variables such as focal length and camera format size. Lenses may also be referred to as being "faster" or "slower" than one another using this same method. A lens with a maximum aperture of is faster than one with an aperture of 5.6, though neither is especially fast. A lens with an aperture of 1.8 is slower than a lens with an aperture of 1.2, though both are fast lenses. The range of lenses considered "fast" has evolved to lower f-numbers over the years, due to advances in lens design, optical manufacturing, quality of glass, optical coatings, and the move toward smaller imaging formats. For example, the 1911 Encyclopaedia Britannica states that "...() are also sometimes classified according to their rapidity, as expressed by their effective apertures, into extra rapid, with apertures larger than 6; rapid, with apertures from 6 to 8; slow, with apertures less than 11." With 35mm cameras, the fastest lenses are typically in the "normal lens" range near 50mm. Longer telephoto designs and wide-angle retrofocus designs tend to be slower. Attaining maximum lens speed requires engineering tradeoffs, and as such, "prime" (fixed focal length) lenses are generally faster than zoom lenses, and modern manual-focus lenses are generally faster than their autofocus counterparts.〔Long, Ben ''Complete Digital Photography'' 2004 Charles River Media〕 The fastest lenses in general production are 1.2 or 1.4, with more at 1.8 and 2.0, and many at 2.8 or slower; 1.0 is unusual, though sees some use, e.g. the discontinued Canon 50mm 1.0, and highly unusual older examples of 0.6, 0.7, 0.8, 0.9 etc. exist, e.g. the Zeiss 50mm 0.7 NASA Kubrick lenses adapted to old film cameras and modern DV cameras. Lens speed also tends to correlate with the price and/or quality of the lens. This is because lenses with larger maximum apertures require greater care with regard to design, precision of manufacture, special coatings and quality of glass. At wide apertures, spherical aberration becomes more significant and must be corrected. However, there are several high-quality fast lenses available that are relatively inexpensive, particularly in normal lens focal lengths. For example, the Canon EF 50mm 1.8 II or Nikon AF Nikkor 50mm 1.8D are very inexpensive, but quite fast and optically well-regarded. Old fast manual focus lenses, just as the Nikkor-S(C) or Nikkor AI-S 50mm 1.4, were historically produced abundantly, and are thus sold relatively inexpensively on the used lens market. ==Fast lenses== For scale, note that 0.5, 0.7, 1.0, 1.4, and 2.0 are each 1 f-stop apart (2× as fast), as an f-stop corresponds to a factor of square root of 2, about 1.4. Thus around 1.0, a change of 0.1 corresponds to about 1/4 of an f-stop (by linear approximation): 1.0 is about 50% faster than 1.2, which is about 50% faster than 1.4. , Canon, Nikon, Pentax and Sony all make an autofocus 50mm 1.4 lens. These are not unusual lenses and are relatively inexpensive. Canon also makes autofocus 50mm and 85mm 1.2 lenses, while Nikon makes a manual focus 50mm 1.2 lens and an autofocus 85mm 1.4; see Canon EF 50mm lenses and Canon EF 85mm lenses for details. Pentax makes a 50mm 1.4 lens and 55mm 1.4 lens for APS-C cameras; see Pentax lenses. Sony makes a 50mm 1.4 lens which is a continuation of the Minolta AF 50mm 1.4 lens, and two lenses with Carl Zeiss: a 50mm 1.4 and 85mm 1.4. In the mid 60s there was something of a fad for fast lenses among the major manufacturers.〔 In 1966 in response to the trend Carl Zeiss displayed a prop lens christened the Super-Q-Gigantar 40mm 0.33 at photokina.〔 Made from various parts found around the factory (the lenses came from a darkroom condenser enlarger) the claimed speed and focal lengths were purely nominal and it wasn't usable for photography. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Lens speed」の詳細全文を読む スポンサード リンク
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