The technical definition of a telephoto lens is one with a focal length greater (or longer) than 50mm (on the 35mm/full frame forma), but the telephoto range is more generally accepted to begin at around 85mm which would be classified as a ‘short telephoto’ lens.
Beyond 300mm you move into the ‘supertelephoto’ range which includes 400mm, 500mm and 600mm lenses. Anything longer than 600mm is generally for specialised applications such as wildlife or some sports.
