Lens speed refers to the maximum 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 a fast lens because it delivers more light intensity (illuminance) to the focal plane, achieving the same exposure with a faster shutter speed. A smaller maximum aperture (larger minimum f-number) is "slow" because it delivers less light intensity 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, 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 f/3.5 is faster than one with an aperture of f/5.6, though neither is especially fast. A lens with an aperture of f/1.8 is slower than a lens with an aperture of f/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 f/6; rapid, with apertures from f/6 to f/8; slow, with apertures less than f/11."
With 35 mm cameras, the fastest lenses are typically in the "normal lens" range near 50 mm. 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.
The fastest lenses in general production are f/1.2 or f/1.4, with more at f/1.8 and f/2.0, and many at f/2.8 or slower; f/1.0 is unusual, though sees some use.
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 f/1.8 II or Nikon AF Nikkor 50 mm f/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 f/1.4, were historically produced abundantly, and are thus sold relatively inexpensively on the used lens market.
Other articles related to "lens speed, lens, speed":
... Nikon, Pentax and Sony all make an autofocus 50mm f/1.4 lens ... while Nikon makes a manual focus 50mm f/1.2 lens and an autofocus 85mm f/1.4 see Canon EF 50mm lenses and Canon EF 85mm lenses for details ... Pentax makes a 77mm f/1.8 lens see Pentax lenses ...
... Further information Lens speed The specifications for a given lens typically include the maximum and minimum apertures, for example, f/1.4–f/22 ... most interest, and is always included when describing a lens ... This value is also known as the lens "speed", because it affects the exposure time ...
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