A telephoto lens is a long focus lens, in which the physical length of the lens is shorter than the focal length. In simple words, a telephoto lens is a long lens that magnifies things that are far away. Generally, all lenses from 100 mm and up are considered as telephoto. The longer the lens, the greater the magnification. That is why, telephoto lens is considered as the best lens for sports, wildlife, portrait and macro photography. It is also used to shoot any object that is very far away or any small object that needs a lot of magnification. Telephoto camera lenses come in both, prime and zoom varieties. Prime has the higher quality and zoom has a greater flexibility.
Telephoto lens can be grouped into three types owing to the huge variations in the focal lengths- Short telephoto lenses (85 mm- 135 mm), are ideal for shooting portraits and candid shots. This lens is compact, lightweight and can be hand held for fast shooting. Medium telephoto lenses (135 mm – 300 mm) are ideal for shooting sports and action.
Aperture is very critical to minimize blur, particularly when shooting fast moving subjects. Super telephoto lenses (300 mm plus) are ideal for shooting wildlife, nature and even sports. Professional photographers usually use this type of lens. Professional photographers use telephoto lens for two main reasons. First reason would be, in order to put backgrounds out-of-focus in the desired photo. To achieve this goal, you will have to use largest aperture and longest lens and get as close as you can to your subject to fill the frame and then take the picture. Second reason to use a telephoto lens would be, to change the perspective of a picture. To achieve this goal, you have to position your subject against the background and use the longest lens. Consider your f-stop.