A lens of greater focal length than a normal lens. Think of the same effect of telescopes or binoculars: a reduced angle of view, and a magnification of what you frame. Normally contrasted with the wideangle lens.

The first and most obvious reason for using a telephoto lens is that you want to take a picture of something small and distant, like a little bird, a naked movie star relaxing in a private beach or some interesting carving on a cathedral spire.

There are are other reasons for using (or not using) a telephoto lens - let me list some (all in comparison to a normal lens):

  1. reduced depth of field: as the lens gets longer, the depth of field becomes smaller, allowing you to exaggerate background fuzzines, and emphasizing the subject. The downside is that it is really easy to take unsharp pictures.
  2. perspective compression: a telephoto lens will give the effect of flattening things. One thing fashion photographers normally want flattened is models' noses (espcially gaunt, anorexic models' noses): that is why they use those monstruous 600 mm lenses.
  3. bulk: despite improvements in design, telephoto lenses are big, heavy, use a lot of glass and are therefore expensive. They look impressive, but are inconvenient as hell, and difficult to use without a tripod.
  4. Working distance: a telephoto forces/allows you maintain a greater working distance from the subject. This can be very good in portraits (because the subject feels less intimidated), in nature photography (because maybe that lion will not eat you) and in fields like industrial photography and photojournalism (do you really want to get close to the drop forge or the riot police ?).
Here are some typical focal lengths in which telephoto lenses are made (this would be for 35mm format):
  • 80 mm: first practical tele, used for portraits.
  • 100 mm: also used for portraits. Typically is between f 1.8 (mega expensive) and f 2.8. Some very nice macro lenses are to be had in this focal distance.
  • 135 mm: an old standby, currently not that popular.
  • 180 mm: this (and the 200 mm) are the last lenses that can be practically used handheld. Nikon makes a very nice 180 mmm f 2.8, with internal focusing and low-dispersion glass.
  • 300 mm: first of the big glass. Exists in f 2.8, much more common in f 4. Here begins the serious wildlife photographer.
  • 400 mm: a popular nature photography lense, made also by many universal (non brand-name) makers.
  • 600 mm: the classic bird lens - birds are small, and easily scared.
The longest tele made is, AFAIK, a 2000 mm f 11 monstruosity produced by Nikon.

Some telephoto lenses, to save on size and weight, employ two mirrors (this is called a catadioptric optical system); they are known as reflex lenses. Fancy telephoto lenses may incorporate an optical image stabilization system.

This writeup was originally a quite irate writeup in answer to another one. The original one having been nuked, I have to rework this into a calmer form.