There are two major groups: converging or positive lenses and diverging or negative lenses.
In particular, positive lenses may be plane convex, biconvex or converging meniscus, while negative lenses may be plane concave, biconcave or diverging meniscus.
The meeting point of the refracted rays (for a converging lens), or their extensions (for a diverging lens), is called the focus of the lens, from which the equivalent focal length (EFL) and back focal length (BFL), key variables in lens design, are derived.
Spherical lenses are characterised by the following parameters: diameter, the measurement that sizes the lens, thickness usually referred to as the thickness at the centre and radii of curvature.
Each radius is the radius of a hypothetical sphere. The accuracy of the bending radii is utterly important, hence measured and monitored. It is indicated with respect to the wavelength lambda, e.g. 2 lambda, lambda/2, lambda/4, etc.
One more key parameter that determines the quality of a lens is centration, understood as the alignment of the spherical surfaces with respect to the optical axis.
The optical axis is the line perpendicular to the two surfaces and passing through their centres of curvature.
We can make any spherical lens, depending on size, performance and type of use.
