Knowing properties of lenses is useful in understanding how the eye works, its defects, and the correction that appropriate lenses can provide.

Knowing properties of lenses is also useful in understanding how cameras work. Photography and cinematography are based on fully realizing the potential of the optical equipment for artistic expression, as Ansel Adams expresses it: `You don't take a photograph, you make it.'

Like any device designed to be used by the eye, the telescope produces outgoing parallel rays of light, which can be focused on the retina by the relaxed eye. Since telescopes are made for viewing distant objects, the incoming rays are also parallel to one another. Thus, a telescope must take incoming parallel rays and convert them to outgoing parallel rays, while creating an image that is magnified. Telescopes, that use only lenses are the