The focal length and principal focus of a lens are defined as follows:

The point where parallel light is brought to a focus is called the PRINCIPAL FOCUS of the lens and the distance of this point from the lens is called the FOCAL LENGTH of the lens.

The focal length is a way of describing the strength of the lens. The shorter the focal length the more sharply curved the sides of the lens will be, and in the case of a convex lens it will converge light more rapidly.

Converging lenses have positive powers, diverging lenses have negative powers.

A lens with a power of + 5 D (dioptres) is a convex lens with a focal length of 20 cm. One with a power of -10 D is a concave lens with a focal length of 10 cm.