Lens speed


Lens speed refers to the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger maximum aperture is called a "fast lens" because it can achieve the same exposure with a faster shutter speed. Conversely, a smaller maximum aperture is "slow" because it delivers less light intensity and requires a slower shutter speed.
A fast lens speed is desirable in taking pictures in dim light, or with long telephoto lenses and for controlling depth of field and bokeh, especially in portrait photography, and for sports photography and photojournalism.
Lenses may also be referred to as being "faster" or "slower" than one another; so an lens can be described as faster than an 5.6.

Tradeoffs

Attaining maximum lens speed requires engineering tradeoffs, and as such, "prime" 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 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 or Nikkor AI-S 50mm 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.

Fast lenses

The fastest lenses in general production now are 1.2 or 1.4, with more at 1.8 and 2.0, and many at 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 "... 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."
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For scale, note that 0.5, 0.7, 1.0, 1.4, and 2.0 are each 1 f-stop apart, 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 : 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 58mm 0.95 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.
The maximum exposure time in free-hand photography can be enhanced even more also for fast lenses, if the camera is equipped with an image stabilisation system. In 2014 Panasonic introduced the fastest lens with in-built stabilisation, the Leica Nocticron 42.5 mm f/1.2, which in the meantime even can be operated with dual image stabilisation, provided that the camera body has an additional stabilising system at the image sensor, too.
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 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, corresponding to a lens with an infinite exit pupil diameter. In practice that cannot be reached due to mechanical constraints of the camera system. Even for systems that can be designed without significant constraints on lens size and image plane distance, the cost of going beyond a numerical aperture of 0.95 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 1.0 to 1.2, with rather strong vignetting towards the edges of the image. Flange distances are significantly smaller for rangefinder and mirrorless cameras, 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 were as follows:
The following camera lenses are no longer in production :
Many very fast lenses exist in C-mount, including:
Very fast lenses in D-mount for 8mm movie use on H8 cameras:
Very fast lenses used in x-ray machines: