PROS AND CONSA teleconverter (or tele-extender) is a short tube that fits between the lens and camera body, and increases the focal length of the lens. Converters come in 1.4x, 1.7x, 2x and even 3x strengths. Attach a 2x converter to a 300mm lens, and you have a 600mm lens. But where the pro 600mm lenses cost upward of $10,000, good 2x converters can be had for under $500.
As an added bonus, adding a teleconverter doesn't change the lens' minimum focusing distance. If you add a 2x converter to a 300mm lens that focuses down to five feet, you get a 600mm lens that focuses down to five feet—close enough to produce a half-life-size magnification at the image plane and good for popular macro subjects, like butterflies and flowers. This is especially nice when you consider that those incredibly expensive 600mm pro lenses won't focus closer than about 15 feet.