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 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, 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 "...[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."
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, e.g. the discontinued Canon 50 1.0, and highly unusual older examples of 0.6/0.7/0.8/0.9 etc. exist, e.g. the Zeiss 50 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 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.
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 2012[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 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 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.
Some of the fastest camera lenses in production as of 2011[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
- Cosina Voigtländer Nokton 17.5mm f/0.95 Aspherical Micro Four Thirds mount
- Cosina Voigtländer Nokton 25mm f/0.95 Micro Four Thirds mount
- Cosina Voigtländer Nokton 42.5mm f/0.95 Micro Four Thirds mount
- Leica Noctilux-M 50 mm f/0.95 ASPH announced on September 15, 2008, it is one of the fastest aspherical lens to have ever reached mass production, with a MSRP of £6290 (approximately US$10,000).
- SLR Magic HyperPrime LM 50mm T0.95 (f/0.92)
- Noktor 50mm f/0.95 'HyperPrime' a fast CCTV lens design adapted for the Micro Four Thirds system
- SLR Magic 25mm T/0.95
- SLR Magic 35mm T/0.95
And as of 2014[update]:
- Mitakon 50mm and 35mm f/0.95
The following camera lenses are no longer in production as of 2010[update]:
- American Optical 81mm f/0.38 Solid Schmidt Mirror lens. Designed for aerial reconnaissance
- GOI CV 20mm f/0.5 Mirror lens
- Signal Corps Engineering 33mm f/0.6
- GOI Iskra-3 72mm f/0.65 Mirror lens
- Fujinon-IDEAX 125mm f/0.67
- Zeiss Planar 50mm f/0.7 Limited production lens built for the NASA space program, used on 35mm movie cameras by Stanley Kubrick for some candlelit scenes in Barry Lyndon
- Tokyo Kogaku Similar 50mm f/0.7, 8 elements in 4 groups, limited produced in 1944 for Japanese Army. (In 1951 another three were produced, two of which were used on a South Pole expedition)
- Kinoptik Lynxar 60mm f/0.7 Reproduction lens, not for photography.
- Wray 64mm f/0.71 Reproduction lens, not for photography.
- LOMO 60mm f/0.75 Reproduction lens, not for photography.
- Aerojet Delft Rayxar 105mm f/0.75 Full Frame aerial photography lens
- Aerojet Delft Rayxar 150mm f/0.75 Medium Format aerial photography lens
- American Optical 43mm f/0.8
- Leica Summar 75mm f/0.85
- Leica Leitz-IR 150mm f/0.85
- Farrand Super Farron 76mm f/0.87
- Farrand Super Farron 150mm f/0.87 Medium Format aerial photography lens
- Рекорд-4 52mm f/0.9
- Nikon TV-Nikkor 35mm f/0.9 Fastest Nikon lens ever made
- Noktor HyperPrime CINE 50mm f/0.92 T0.95, Fastest cinema lens made for 35mm interchangeable lens camera
- Canon 50mm f/0.95 Available in TV and Canon 7 Rangefinder Version
- Astro Berlin 52mm f/0.95
- 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
- Panavision 50mm f/1.0
- Nikkor-O 50mm f/1.0 Prototype lens for Nikkor-S Rangefinder camera
- Nikkor-N 50mm f/1.1 lens for Nikkor-S Rangefinder camera
- Noct-Nikkor 58mm f/1.2
- Leica ELCAN 90mm f/1.0
- Wild Heerbrugg Reconar 98mm f/1.0 Medium Format aerial photography lens
- Kollmorgen 153mm f/1.0
- Zeiss UR 250mm f/1.0
- Canon 8.5–25.5 mm 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.
- Fujinon 50mm f/0.7
- Canon 'TV-16' 25mm f/0.78
- Apollo 25mm f/0.85
- Ernitec 25mm f/0.85
- Fujinon 25mm f/0.85
- Tarcus 25mm f/0.85
- Kern Switar 18mm f/0.9 built for NASA for Apollo Moon landing 
- Ampex 'LE610 Television Lens' 25mm f/0.95
- Angenieux 'M1' and 'M2' 25mm f/0.95 (M1 was a consumer product, while M2 was aimed at the professional cine market)
- Angenieux 'M2' 28mm f/1.1
- Angenieux 35mm f/0.95
- Angenieux 50mm f/0.95 Type M1 (original more common), and Type M2 (better corrected for aberrations and distortions, designed for NASA, very rare and hard to find)
- AstroScope 25mm f/0.95
- Avenir 25mm f/0.95
- Century 'Nighthawk' 25mm f/0.95
- Carl Meyer 25mm f/0.95
- Cinetar 25mm f/0.95
- Goyo Optical 17mm, 25mm, and 50mm f/0.95
- JML 25mm and 50mm f/0.95
- Navitar 25mm and 50mm f/0.95
- Navitron 25mm and 50mm f/0.95
- Schneider Kreuznach 'Xenon' 17mm, 25mm, and 50mm f/0.95
- Senko 25mm and 50mm f/0.95
- Soligor 'Super Elitar' 25mm f/0.95
- Som Berthiot 'Cinor' 25mm and 50mm f/0.95
- Tarcus 'I.T.V. Lens' 50mm f/0.95
- Precise Optics 50mm f/0.95
- Kowa 50mm f/0.95
- Yakumo 25mm and 50mm f/0.95
- Zeika 'Nominar' 25mm f/0.95
- Kaligar 'Nominar' 25mm f/0.95
- Dallmeyer 25mm f/0.99 (1930)
- Astro Berlin 25mm f/1.0
- Astro Berlin 'Tachonar' 35mm and 75mm f/1.0
- Carl Meyer 38mm f/1.0
- RTH (Rank/Taylor Hobson) Monital 130mm f/1.0 made by SOPELEM in France
Very fast lenses in D-mount for 8mm movie use on H8 cameras:
- Kern Switar 13mm f/0.9
- Cinetor 'TELE-PHOTO' 37.5mm f/1.0
- Walz 'TELEPHOTO' 37.5mm f/1.0
- Amitar 'Telephoto' 38.1mm f/1.0
- Rexer 'TELE' 38mm f/1.0
- Manon 'Telephoto' 37.5mm f/1.0
Very fast lenses used in x-ray machines:
- Zeiss R-Biotar 100mm f/0.73
- LOMO 100mm f/0.73
- Canon 50mm/65mm f/0.75
- Leitz 50mm/65mm f/0.75
- Rayxar 50mm/65mm/150mm f/0.75
- Tachon Astro-Berlin 65mm f/0.75
- Rodenstock XR-Heligon 42mm/50mm f/0.75
- Rodenstock XR-Heligon 68mm f/0.95 etc.
- Kowa 42mm/65mm f/0.75
- Carl Zeiss Jena 50mm f/0.77
- Kowa 55mm f/0.8
- Zeiss R-Biotar 55mm f/0.85
- Lenzar Optics 184.6mm f/0.9 (Photographic lens made by Lenzar Optics Corp., Riviera Beach FL, f0.9–f8)
- Kowa 33.5mm f/0.95
- Kowa 55mm f/1.0
- Rodenstock Heligon 68mm f/1.0
- Canon 90mm f/1.0
- Fuji 90mm f/1.0
- Kowa 90mm f/1.0
- Leitz 90mm f/1.0
- Mt Prospect 90mm f/1.0
- Zeiss R-Biotar 120mm f/1.0
- Waldren, Margaret (and others) Advanced Digital Photography 2004 Media Publishing
- Long, Ben Complete Digital Photography 2004 Charles River Media
- Maiello, Agostino (January 2000). "L'OBIETTIVO PIU' LUMINOSO DEL MONDO". Nadir.it (in Italian). Nadir Magazine. Retrieved 28 September 2013.
- Zhang, Michael (Aug 6, 2013). "Carl Zeiss Super-Q-Gigantar 40mm f/0.33: The Fastest Lens Ever Made?". Petapixel. Retrieved 28 September 2013.
- "Cosina Voigtänder — Nokton 17.5mm F0.95 Aspherical". Cosina Voigtländer.
- "Cosina Voigtänder - Nokton 25mm F0.95". Cosina Voigtländer. 2010-08-26. Retrieved 2013-12-03.
- The Voigtlander 17.5mm f/0.95 at B&H Photo
- The Voigtlander 25mm f/0.95 at B&H Photo
- The Voigtlander 42.5mm f/0.95 at B&H Photo
- "Leica offers World's fastest Aspherical lens". Leica Camera. 2008-09-15. Retrieved 2009-02-14.
- "NOKTOR – Ultra Fast Lenses". Retrieved 2010-05-26.
- "KenRockwell.com – Noktor 50mm f/0.95". Retrieved 2011-11-22.
- "USAF Lens Datasheets - Type 1 Aerial Reconnaissance". archive.org. Retrieved 4 October 2013.
- Two Special Lenses for "Barry Lyndon", by Ed DiGiulio (President, Cinema Products Corp.), American Cinematographer
- Lossau, Jürgen (2003). The Complete Catalogue Of Movie Cameras, Hamburg/Germany, atoll medien, p. 59, ISBN 3-9807235-3-4
- "Science, Kern-Paillard advertisement" (PDF) 165.
- Just how fast is that lens? f-number doesn't directly determine focal-plane illuminance, due to light transmission losses.