A lens is an optical device consisting of one or more transparent elements made from optical glass or high quality plastic that is capable of refracting - bending - light rays passing through it in a controlled and predictable manner, converging or diverging them to form a focused image of a subject on the film, image sensor, or other light sensitive surface positioned at the appropriate distance behind it. The lens is the fundamental optical component of any camera system, and its design, construction, and optical quality have a profound influence on every aspect of the images it produces, from sharpness and resolution to colour rendition, contrast, and the character of out of focus areas.
The simplest possible lens is a single element - a piece of glass or plastic ground and polished to a precise curved profile on one or both surfaces. A convex or positive element, which is thicker at its centre than at its edges, converges parallel light rays to a focal point at a specific distance behind the element known as the focal length. A concave or negative element, thinner at its centre than at its edges, diverges light rays and is used in combination with positive elements to correct optical aberrations and control the overall optical behaviour of the lens system. The vast majority of photographic lenses are compound lenses - complex optical systems comprising multiple individual elements of varying shapes, thicknesses, and optical properties, arranged in groups along a common optical axis to collectively produce a focused, corrected, and high quality image across the full image circle.
The use of multiple elements in a photographic lens serves several essential optical purposes. Different wavelengths of light are refracted by different amounts when passing through glass - a phenomenon known as dispersion - causing colours to focus at slightly different distances and producing chromatic aberration in the image. By combining elements made from different types of optical glass with different dispersive properties - known as crown and flint glass - lens designers can engineer achromatic or apochromatic corrections that bring multiple wavelengths of light to a common focus, minimising colour fringing and producing images with accurate, neutral colour rendition. Multiple elements also allow designers to correct a range of other optical aberrations inherent in simple single element lenses, including spherical aberration, coma, astigmatism, field curvature, and distortion, each of which degrades image quality in a different way and requires specific optical strategies to minimise.
Modern photographic lenses may contain anywhere from a handful of elements in a simple prime lens design to twenty or more elements in complex zoom lenses, macro lenses, and specialist optical designs. Advanced optical glass formulations including extra low dispersion glass, fluorite elements, and aspherical elements - ground or moulded to a non-spherical profile that reduces spherical aberration more effectively than conventional spherical elements - are used in premium lens designs to achieve exceptional optical performance across the full aperture range and focal length range of the lens. Anti-reflective coatings applied to the surfaces of each element minimise the internal reflections that cause flare and reduce contrast, ensuring that the maximum possible amount of light passes cleanly through the full optical system to reach the film or sensor.
Lenses are available in an enormous variety of focal lengths and designs, from ultra wide angle fish-eye lenses covering 180 degrees or more, through standard lenses approximating the perspective of human vision, to extreme telephoto lenses capable of bringing distant subjects dramatically closer. Zoom lenses offer a continuously variable focal length within a specified range, while prime lenses of fixed focal length typically offer superior optical quality, wider maximum apertures, and more compact designs. Specialist lens types including macro lenses for close up photography, tilt-shift lenses for perspective control, and catadioptric or mirror lenses combining refractive and reflective optical elements extend the capabilities of the photographic lens further still.