The surfaces of the lenses are spherical and therefore turn the parallel light beam passing through them into a converging (if it is a convex lens) or a diverging light beam (if it is a concave lens).

The line connecting the centers of the spheres forming the surfaces of the lens is called the main optical axis of the lens. The optical center of the lens is the point located in the center of the lens on the principal optical axis.

The main optical axis of optical systems is an imaginary line on which the centers of all lenses and mirrors used in the system should be located in order for light to pass through them correctly. When performing our experiments, the optical axis is an auxiliary line for placing the object, lenses and other optical elements.

The focus $F$ of a convex lens is called the point where the parallel light beam falling on the lens is focused. The focal length $f$ is the distance between the focus and the optical center of the lens.

In the case of a concave lens, the point at which the extensions of the scattered rays of the parallel light beam passing through the lens are called the focus of the lens. Since the extensions of the scattered rays converge on the other side of the lens, the focal length of a concave lens is conventionally minus.

The optical power of a lens $\left(D\right)$ is the reciprocal of the focal length of the lens:

Therefore

An optical system consisting of two or more closely spaced lenses is called a compound lens.

The optical power of a compound lens is equal to the sum of the optical powers of the individual lenses:

For example, if a compound lens consists of two convex lenses with optical powers of $2dpt$ and $3dpt$, respectively, then the optical power of the compound lens consisting of these lenses is $5dpt$.