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 (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. 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. 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 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 '... 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, 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.