The distance from the focal point to the lens is defined to be the focal length of the lens. Parallel rays can be made to enter from the other side of the lens, too, so we can find a focal point on either side of the lens. Each lens has two focal points and two focal lengths. If the lens is thin compared to its focal lengths, the two focal lengths are approximately equal in size. This is the most familiar case.
Lenses are usually symmetric about an axis, called the lens axis .
For a single-lens system, this axis is also called the optical axis . A converging lens is said to have positive focal length. A converging lens causes exiting rays to be more convergent coming out than they were entering the lens. A diverging lens is said to have negative focal length. A diverging lens can form a real image or a virtual image of a real object.