A Teleconverter is an optical device
that goes between your camera and lens to increase the focal
length. There are usually two lengths available - 1.4X and 2X. The number
is a multiplier. For example, a 2X teleconverter on
installed on a 105mm lens results in a 210mm focal length. A teleconverter is an inexpensive way to get a new effective lens focal length!
Like most things in life, nothing is free - the 2X teleconverter
sucks up two additional stops of light, turning a 105mm f2.8 into an
f5.6. Also, the longer focal length means that you will
need a faster shutter speed, which will make hand-holding the lens with teleconverter in low light more difficult.
A teleconverter is a relatively
cheap way to get a longer focal length. Just make sure to
take into account the loss of light to be sure it will still
be useful to you.
Compatibility of the Nikon TC-200:
The TC-200 is compatible with almost any Nikon AI or AIs compatible lens up to 200mm in focal length. It does not relay autofocus information from an AF lens, so while you can attached it to a modern body and lens, autofocus will not work. The TC-200 is made up of 7 elements in 5 groups.