Lens speed is a term used to describe the maximum aperture of a photographic lens - the largest opening to which the iris diaphragm can be set, expressed as the smallest available f-number for that lens. A lens with a large maximum aperture that admits a great deal of light is described as a fast lens, while one with a smaller maximum aperture that admits less light is described as a slow lens. The terms fast and slow derive from the fact that a larger aperture allows a correspondingly faster shutter speed to be used for a given exposure, enabling the photographer to shoot in lower light conditions or freeze motion more effectively than would be possible with a slower lens of equivalent focal length.
The speed of a lens is determined by the ratio of the diameter of the entrance pupil - the effective aperture opening as seen from the front of the lens - to the focal length of the lens, which is the fundamental optical relationship that defines the f-number scale. A lens with a wider entrance pupil relative to its focal length has a lower f-number and is therefore faster, while a lens with a narrower entrance pupil relative to its focal length has a higher f-number and is slower. This relationship means that achieving a given maximum aperture becomes progressively more physically demanding and optically complex as focal length increases, as the entrance pupil must be correspondingly larger to maintain the same f-number.
The concept of lens speed is therefore always relative to focal length, and cannot be meaningfully assessed from the f-number alone without considering the focal length of the lens in question. A 50mm lens with a maximum aperture of f/1.4 is considered an exceptionally fast standard lens, as the large aperture is achievable at this relatively modest focal length without requiring an impractically large front element. A 400mm telephoto lens with a maximum aperture of f/3.5, while representing a much smaller physical opening in absolute f-number terms than an f/1.4 standard lens, is considered an extremely fast telephoto because achieving even f/3.5 at 400mm requires a very large and expensive optical design with a substantial front element diameter. Conversely, a 28mm wide angle lens with a maximum aperture of f/3.5 is considered relatively slow for its focal length, as the short focal length makes large aperture designs comparatively straightforward to engineer, and many 28mm lenses are available with maximum apertures of f/2.8, f/2, or even f/1.4.
Fast lenses command a significant premium in terms of both cost and physical size compared to slower lenses of equivalent focal length and optical quality, as the larger optical elements, more complex aberration correction requirements, and more demanding manufacturing tolerances associated with large aperture designs add considerably to the complexity and expense of production. Despite this, fast lenses are highly valued by photographers working in low light environments, those who regularly shoot action and sports subjects requiring fast shutter speeds, and those who use shallow depth of field creatively to isolate subjects against softly blurred backgrounds - a quality of image rendering that is only achievable with lenses of large maximum aperture.