What they Do
A teleconverter is a magnification lens that goes between a camera lens and a camera body. Teleconverters generally only work with specific lenses inside a manufacturer's line, so it's important to check the lens compatibility with the teleconverter before investing in one. Teleconverters are useful for extending the focal length of a lens without having to invest in a new one.
Types
Teleconverters are usually available in three different magnification factors: 1.4, 1.7, and 2x. Using one of these teleconverters increases the focal length of the lens it is put on by that factor. For example, a 70-200 f2.8 VR Nikon lens with a 2x teleconverter on it becomes a 140-400 lens. Since teleconverters are compact, it is an excellent way to get greater focal distance on your camera without having to carry around extra, heavier, lenses.
Limitations
Teleconverters generally work best on faster lenses, such as those that are f2.8 and better. One reason is that putting a teleconverter on a lens reduces the aperture by the amount of magnification on the teleconverter. For instance, in the aforementioned example, the f2.8 70-200 lens with the 2x teleconverter becomes a 140-400 lens with an f5.6 aperture. As long as the photographer can compensate for this decrease in aperture size, using a teleconverter is an excellent tool.
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