Telephoto lenses are used when you wish to get closer to the subject to get the subject larger in the frame. The image above was taken at an air show and a 300mm telephoto lens was used to help fill the frame with the subject and the smoke trails.
Teleconverters are also very usefull to increase the focal length of the lens, they effectively multiply the focal length of the lens in use. Teleconverters are generally available with multiplication factors of x1.4, x1.7 or x 2.0, however they do have some disadvantages in that they reduce the f stop of the lens and they can also cause some degradation in the final image.
Telephoto lenses can be very usefull in some of the following applications:
- Airshows
- Bird photography
- Animal photography
- Close up portraits
- Insects
- Motor cycle or car racing
- Athletics and other sporting events
Generally any lens longer than 50mm is considered as a telephoto lens. Typically a lens of 85-135mm would be suitable for portraits. Lenses of 200-300mm are suitable for animals and lenses over 300mm are usefull mainly for photography of wild birds.