Supertelephoto lenses are really expensive, but worth it for those who can afford the cost, due to their superb optics and AF performance, and fast maximum apertures. But there’s an accessory that can give those on a budget access to supertele "reach" at a much lower cost: the teleconverter.
PROS AND CONS
A 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.
Converters do have their drawbacks, of course. For one thing, they reduce the light transmitted to the image sensor (and SLR viewfinder) by 1 stop for a 1.4x converter, by 1.5 stops for a 1.7x, by 2 stops for a 2x and by 3 stops for a 3x converter. Built-in TTL metering automatically will compensate for this, but it means you’ll be shooting at a slower shutter speed or a higher ISO setting when you use a converter. With the amazing high-ISO performance of today’s DSLRs, this isn’t the problem it was with film.