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.