A positive lens is an optical element that is thicker at its centre than at its edges - a convex form - which causes parallel rays of light passing through it to converge towards a common focal point located on the far side of the lens at a specific distance behind it known as the focal length. This converging behaviour is the defining optical characteristic of a positive lens, and it is the fundamental principle upon which the image forming capability of virtually all photographic lenses is based. Because a positive lens brings light rays together to form a real, inverted image of a subject at the focal plane, it is also referred to as a converging lens.
Positive lenses are available in several geometric forms, all sharing the common characteristic of being thicker at the centre than at the periphery. Biconvex lenses are convex on both surfaces, with the centre being thicker than the edges on both faces. Plano-convex lenses are flat on one surface and convex on the other, with the centre thicker than the edges on the convex side. Converging meniscus lenses have one convex and one concave surface, with the convex surface having a greater curvature than the concave, resulting in an overall converging power despite having one concave face. Each of these geometric forms produces slightly different optical aberration characteristics and is suited to different positions and functions within a compound lens design.
The focal length of a positive lens - the distance from the lens to the point at which parallel rays from a distant subject converge to a focus - is determined by the curvatures of its surfaces and the refractive index of the glass from which it is made. A more strongly curved lens with steeper surface profiles will have a shorter focal length and stronger converging power, while a more gently curved lens will have a longer focal length and weaker converging power. The refractive index of the glass affects the focal length independently of the surface curvature - a higher refractive index glass allows the same focal length to be achieved with gentler surface curvatures, which is advantageous for controlling certain optical aberrations.
In photographic lens design, positive elements are the primary image forming components, providing the converging power that brings the light from the subject to a focus at the film or sensor plane. They are used in combination with negative elements, which diverge light rays and are used to correct the various optical aberrations that positive elements alone would produce, to create compound lens designs of high optical quality across a range of apertures and focusing distances. The interaction between positive and negative elements of different optical glass types is the fundamental design tool through which lens designers achieve the correction of chromatic aberration, spherical aberration, field curvature, distortion, and other optical defects that would compromise image quality if left uncorrected.