A negative lens is an optical element that is thinner at its centre than at its edges - a concave form - which causes parallel rays of light passing through it to diverge away from the optical axis rather than converging towards a focal point as a positive convex lens would. Because the refracted rays diverge after passing through a negative lens, they do not converge to form a real focal point on the far side of the lens. Instead, when the diverging rays are traced back along their apparent paths, they appear to originate from a virtual focal point located on the same side of the lens as the incoming light, giving the negative lens a negative focal length - the property from which it takes its name.
Negative lenses are available in several geometric forms, all sharing the common characteristic of being thinner at the centre than at the periphery. Biconcave lenses are concave on both surfaces, while plano-concave lenses are flat on one side and concave on the other, and convexo-concave or meniscus lenses have one convex and one concave surface, with the concave surface having a greater curvature than the convex, resulting in an overall diverging power despite having a convex face.
In photographic lens design, negative lens elements are rarely used in isolation but play an essential and indispensable role within complex multi-element compound lens systems, where they are combined with positive elements to achieve optical corrections and design goals that neither type of element could accomplish alone. The most fundamental application of negative elements in compound lens design is the correction of chromatic aberration - the tendency of a simple positive lens to focus different wavelengths of light at slightly different distances, producing colour fringing in the image. By combining a positive element made from one type of optical glass with a negative element made from a glass of different dispersive properties, lens designers can create an achromatic doublet or more complex apochromatic correction in which two or more wavelengths of light are brought to a common focus, substantially reducing chromatic aberration and improving colour fidelity.
Negative elements are also used to control field curvature, coma, astigmatism, and other optical aberrations, and are a key component in telephoto lens designs where a negative rear group is used to extend the effective focal length of the lens beyond its physical length - allowing long focal length telephoto lenses to be made considerably more compact than their focal length alone would otherwise require. Similarly, retrofocus and reverse telephoto wide angle lens designs use a negative front group to increase the back focus distance of the lens, ensuring that the rear element clears the mirror box of a single lens reflex camera even at wide angle focal lengths where a simple symmetrical lens design would require the rear element to be positioned impractically close to the film or sensor plane.