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A fast prime (fixed focal length) lens, the Canon 50mm f/1.4 (left), and a slower zoom lens, the Canon 18-55mm f/3.5-5.6 (right); this lens is faster at 18mm than it is at 55mm.
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 called a "fast lens" because it can achieve the same exposure with a faster shutter speed. Conversely, a smaller maximum aperture (larger minimum f-number) is "slow" because it delivers less light intensity and requires a slower (longer) shutter speed.
Lenses may also be referred to as being "faster" or "slower" than one another; so an f/3.5 lens can be described as faster than an f/5.6.
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.
With 35mm cameras, the fastest lenses are typically in the "normal lens" range near 50mm and there are several high-quality fast lenses available that are relatively inexpensive. For example, the Canon EF 50mm f/1.8 II or Nikon AF Nikkor 50mm 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.
Especially outside of the "normal lenses", 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. Faster telephoto and wide-angle retrofocus designs tend to be much more expensive.
The fastest lenses in general production now 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. What is 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 Encyclopædia Britannica states that "...[Lenses] 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."
For scale, note that f/0.5, f/0.7, f/1.0, f/1.4, and f/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 f/1.0, a change of 0.1 corresponds to about 1/4 of an f-stop (by linear approximation): f/1.0 is about 50% faster than f/1.2, which is about 50% faster than f/1.4.
As of 2017[update], Canon, Nikon, Pentax and Sony all make an autofocus 50mm f/1.4 lens. These are not unusual lenses and are relatively inexpensive. Canon also makes autofocus 50mm and 85mm f/1.2 lenses, while Nikon makes a manual focus 58mm f/0.95 lens and an autofocus 85mm f/1.4; see Canon EF 50mm lenses and Canon EF 85mm lenses for details. Pentax makes a 50mm f/1.4 lens and 55mm f/1.4 lens for APS-C cameras; see Pentax lenses. Sony makes a 50mm f/1.4 lens which is a continuation of the Minolta AF 50mm f/1.4 lens, and two lenses with Carl Zeiss: a 50mm f/1.4 and 85mm f/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 f/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.
Maximum possible speed
Theoretically, the smallest f-number is 0 (or numerical aperture of 1), corresponding to a lens with an infinite exit pupil diameter. In practice that cannot be reached due to mechanical constraints of the camera system (shutter or mirror clearance, mount diameter). Even for systems that can be designed without significant constraints on lens size and image plane distance (e.g. microscopy and photolithography systems), the cost of going beyond a numerical aperture of 0.95 (f/0.164) is prohibitive with respect to the minor gain in NA.
In SLR camera systems, typical mount diameters are in the range of 44-54 mm, with flange distances around 45 mm. This limits the maximum possible f number to f/1.0 to f/1.2, with rather strong vignetting towards the edges of the image. Flange distances are significantly smaller for rangefinder and mirrorless cameras (even below 20 mm), theoretically enabling designs down to something like f/0.7 or even faster. The chance of seeing these in practice will be slim, since their practical use is limited, and the cost and weight are likely not competitive with respect to equivalent imaging solutions employing larger sensors.
List of ultrafast lenses
Some of the fastest camera lenses in production as of 2017[update] were as follows:
Handevision Ibelux 40mm f/0.85, Made for multiple camera mounts including Micro 4/3, Sony E-Mount and Fujifilm X-Mount
Perkin Elmer 114mm f/0.95 Medium Format aerial photography lens
Pacific Optical 25mm f/1.0 Medium Format Fish-eye lens. Only 3 were ever made for the Canadian Government for aurora borealis research in the late 60s/early 70s. One of these lenses was used in the production of the IMAX movie Solarmax
Leica Noctilux 50mm f/1.0 Leica M mount, discontinued and replaced 2008 with a new Noctilux, see above
Canon EF 50mm f/1.0 for Canon autofocus SLR, now out of production
Canon 8.5-25.5mm f/1.0 zoom lens, made 1975-1983 for the 310XL Super 8mm silent and sound camera series, fastest lens ever made in Super8, was originally advertised as facilitating "shooting at candlelight" in combination with 160-ASA films.
Many very fast lenses exist in C-mount (such as used by 16mm film cameras, CCTV, medical & scientific imaging systems), including: