The Collaborative International Dictionary of English v.0.48:

Telephoto lens \Tel`e*pho"to lens\ n.
   a special compound camera lens with a long effective focal
   length but used in a camera with a short focal length,
   allowing large images to be obtained of distant objects when
   used in a camera in place of an ordinary lens; -- called also
   telephotographic lens.
   [PJC]