Many photography teachers talk about “zooming with your feet” by walking closer to and further from your subject instead of purchasing large and expensive zoom lenses. Sometimes, however, this is simply not possible and a longer focal length is needed to capture a particular scene. A teleconverter can help in these situations by increasing the effective focal length of a lens.
A teleconverter lens is mounted directly on the camera, and the lens is mounted on the teleconverter. They usually come with multiplication factors of 1,4x, 1,7x, or 2x and increase the focal length by this factor.
But teleconverters also reduce the effective aperture by the same ratio, so an f4 lens will become an effective f8 lens with a 2x teleconverter (two stops). Teleconverters also magnify any aberration created by the main lens. However, sometimes using a teleconverter is the only way to capture a specific scene adequately.
Teleconverters and lenses
Current teleconverters can communicate seamlessly with the lens and camera, particularly when they are made by the same manufacturer. Older models sometimes prevent auto exposure and auto-focus, and not all teleconverters work well with all lenses and can even slow down auto focus speed.
Teleconverter lenses are most useful on fast lenses, or when taking photos in bright light. They generally work better on fixed focal length lenses than zoom lenses.