The telephoto lens is a specific form of photographic lens in which the effective focal length is less than the actual measured focal length of the lens. Such a lens will produce the same results as a lens with a longer measured focal length.
For example a long focus lens could have a 500mm focal length. However, a half meter long lens is impractical. A shorter length lens, with the same effective focal length is lighter, easier and more convenient to use.
Telephoto lens elements
A specialised group of lens elements creates a light path inside the lens which mimics the actual focal length but is shorter. These are referred to as the telephoto elements or telephoto group(s) (of elements).
The telephoto elements may create an effective focal length which exceeds the actual measured length of the lens body. In effect this is a long focal length inside a short lens body.
Telephoto elements may be included in both fixed focal length prime lenses and zooms. A photographic lens with telephoto elements is referred to as a telephoto lens or telephoto zoom.
There are optical penalties for using the ‘telephoto’ lens grouping. Increased aberrations and reduced sharpness are two common effects. However, the high quality of lens manufacturing and additional in-camera processing to remove aberrations have improved the resultant image quality.
Telephoto lens relationship to zoom lenses
Some people might assume a prime lens is a shorter lens. The reasoning is that there are less lens elements in a prime lens. It is not true. The actual length of a lens is more closely related to the optical path length of the light through the lens to the digital image sensor. A telephoto lens tends to have a shorter body. This is because true focal length is mimicked by a shorter focal path in the telephoto element(s). This can mean that a telephoto lens that is also a zoom lens may be shorter than a prime lens of the same focal length.