A telephoto lens is simply a long lens that magnifies things that are far away.
I often find when I teach my online beginning photography class that students confuse the terms “telephoto lens” and “zoom lens.” This misconception is likely caused by point-and-shoot cameras that offer a “10x zoom.” In this sense, the zoom actually refers to the amount of magnification range the lens is capable of (from a wide shot to a tight shot, for example), and not the total magnification power of the lens. Zoom lenses can actually be fairly wide or they can magnify the image greatly.
Generally, all lenses from 100mm and up are considered telephoto. The longer the lens in terms of millimeters, the greater the magnification of the photo. Therefore, a telephoto lens is the best lens for shooting sports photography, wildlife, or any other object that is far away or which is small and needs a lot of magnification.
As I have already done twice in this post, photographers often use the term “long lens” when they really mean “telephoto lens.” Although this is common for photographers to refer to a telephoto lens this way, it is not technically correct. While it is true that longer lenses are generally more telephoto (have greater magnification), it is not always the case. In fact, some telephoto lenses are even shorter in size than some wide angle lenses. The only way to truly know how much magnification a lens will produce is to check the focal range of the lens by looking for the magnification in millimeters. Again, anything over 100mm is considered telephoto.
I often find that photographers who get their first DSLR are often fixated on purchasing the longest telephoto lens available. This is a natural thought because most of us have been disappointed by point-and-shoot cameras that are too wide and do not allow the photographer to crop in tight on what he or she is photographing; however, most photographers eventually find that the longest focal length needed for landscape photography is only 24mm, and the longest focal length that a portrait or people photographer commonly uses is 200mm. Unless you shoot sports, macro, wildlife, or do some other specialized type of photographic work, it is unlikely that you will need an extremely long telephoto lens.
When looking at the many different telephoto lenses available, keep in mind that the magnification of the lens is not equal at the long end as it is on the short end. For example, if you use a 300mm lens and take a picture and then take the same picture with a 310mm lens, the magnification of the second photo will hardly be any different than that of the 300mm lens. However, at lower focal lengths, a 10mm difference in the lens makes a dramatic difference in the magnification of the lens.
One last consideration when determining how the focal length of a lens will impact the magnification is whether you are shooting from a crop sensor or full frame camera. See the difference in focal length between crop and full frame cameras here.
However, if you are in the market for purchasing a telephoto lens, check out my lens recommendations to find the best lenses currently available for your Canon or Nikon DSLR.
If you are a new photographer, I would really encourage you to check out my free photo basics series. It is a collection of 5 blog posts that walks you through the most important elements of great photography. A lot of my readers really like that series, so be sure to check it out.