Lens elements are the individual pieces of precision optical glass - or in some cases optical plastic or specialist crystalline materials - that together form the complete optical system of a photographic lens. Each element is a discrete optical component, ground and polished to a precise curved profile on one or both of its surfaces and manufactured to exacting standards of optical clarity, refractive index consistency, and surface accuracy. It is the combined refractive effect of all the elements working together in their carefully calculated arrangement that bends and directs light rays passing through the lens to form a sharp, well corrected, and accurately rendered image on the film or image sensor at the focal plane.
The number of elements in a photographic lens varies considerably depending on the complexity of the optical design and the performance requirements of the lens. Simple fixed focal length lenses for basic cameras may contain as few as three or four elements, while standard prime lenses for SLR and mirrorless cameras typically contain between six and nine elements. Complex zoom lenses, macro lenses, and high performance telephoto designs may incorporate fifteen, twenty, or even more individual elements to achieve the level of aberration correction, zoom range coverage, and overall optical performance required of a modern professional lens. As a general principle, more complex lens designs with wider zoom ranges or more demanding performance specifications require more elements to achieve adequate optical correction across their full operating range.
Lens elements are arranged within the barrel in groups - collections of two or more elements that are cemented together or held in close proximity and function as a combined optical unit within the overall design. The grouping of elements serves several purposes, including the correction of chromatic aberration through the pairing of positive and negative elements made from different types of optical glass with complementary dispersive properties, and the reduction of the overall physical length of the lens by combining the refractive power of multiple surfaces into a more compact arrangement. The optical specification of a lens is commonly expressed in terms of both the total number of individual elements and the number of groups into which they are arranged - for example, a lens described as having six elements in four groups might consist of two individual single elements and two cemented pairs, each pair counting as a single group despite containing two elements.
The optical glass used for individual lens elements is available in a wide variety of formulations, each with specific refractive index and dispersion characteristics that lens designers exploit to achieve particular optical corrections and performance goals. Standard optical crown and flint glasses form the basis of most conventional lens designs, while specialist materials including extra low dispersion glass, anomalous dispersion glass, fluorite crystal elements, and aspherical elements ground or moulded to a non-spherical surface profile are incorporated into premium and high performance lens designs to achieve superior correction of chromatic aberration, spherical aberration, and other optical defects that limit the ultimate image quality of designs using conventional glass types alone.