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 a fast lens because it delivers more light intensity (illuminance) to the focal plane, allowing 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,^[1] 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.^[2]

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. This is not a hard and fast rule, however, as 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 is very inexpensive, but quite fast and optically well-regarded.

Fast lenses

Canon 85mm f/1.8 and f/1.2 showing their large entrance pupils

For scale, note that 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 correspond 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 2010^[update], Canon, Nikon and Pentax 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 77mm f/1.8 lens; see Pentax lenses.

For historical background, see (Puts 2009).

Some of the fastest camera lenses currently in production as of 2011^[update] are as follows:

• Cosina Voigtländer Nokton 25mm f/0.95 (Micro Four Thirds mount, announced 26/8/2010^[3]
• Leica Noctilux-M 50 mm f/0.95 ASPH announced on September 15, 2008, it is the fastest aspherical lens to have ever reached mass production, with a MSRP of £6290 (aprox $10,000USD.^[4]
• Noktor 50mm f/0.95 'HyperPrime'^[5] a fast CCTV lens design adapted for the Micro Four Thirds system^[6]

The following camera lenses are no longer in production as of 2010^[update]:

• Signal Corps Engineering 33mm f/0.6
• Zeiss 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)^[7]
• Tokyo Kogaku 50mm f/0.7 (WWII) and Similar 5cm f/0.7 (1951; only three were produced, two of which were used on a South Pole expedition)
• Wray 64mm f/0.71
• Leica Leitz 75mm f/0.85
• Farrand Super Farron 76mm f/0.87
• Nikon TV-Nikkor 35mm f/0.9 Fastest Nikon lens ever made
• SLR Magic 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
• Perkin Elmer 114mm f/0.95 Military lens for Medium Format photography
• Leica ELCAN 90mm f/1.0
• 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 8.5-25.5 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.^[8]

Many very fast lenses exist in C-mount (such as used by CCTVs), including:

• Fujinon 50mm f/0.7
• Canon 'TV-16' 25mm f/0.78
• Apollo 'High Speed Lens' 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)^[9]
• Ampex 'LE610 Television Lens' 25mm f/0.95
• Angenieux 'M1' and 'M2' 25mm f/0.95 NASA used the M1 for first high-resolution photographs of the Moon by Ranger 7)
• Angenieux 50mm f/0.95
• 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
• Yakumo 25mm and 50mm f/0.95
• Zeika 'Nominar' 25mm f/0.95
• Dallmeyer 25mm f/0.99 (1930)
• Astro Berlin 25mm f/1.0
• Astro Berlin 'Tachonar' 35mm 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

Very fast lenses used in x-ray machines:

• 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
• Kowa 42mm/65mm f/0.75
• Kowa 55mm f/0.8
• 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

References

External links

• Just how fast is that lens? f-number doesn't directly determine focal-plane illuminance, due to light transmission losses.

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• Tokyo Kogaku 50mm f/0.7 (WWII) and Similar 5cm f/0.7 (1951; only three were produced, two of which were used on a South Pole
• Wray 64mm f/0.71
• Leica Leitz 75mm f/0.85
• Farrand New York Super Farron 76mm f/0.87
• Nikon TV-Nikkor 35mm f/0.9 Fastest Nikon lens ever made
• Canon 50mm f/0.95 Available in TV and Canon 7 Rangefinder Version
• Perkin Elmer 114mm f/0.95 Military lens for Medium Format photography
• Leica ELCAN 90mm f/1.0
• 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 8.5-25.5