Right: A teleconverter fits between your lens and camera body, and unlike extension tubes, it has optical elements that create the magnification. Because of the optics involved, matching appropriate lenses to converters is a good idea. Left: Examples of teleconverters from Sigma, Canon, Tamron, Pro-Optic, Nikon, Kenko and Olympus.
A really long lens will fill the frame with the subject, but really long lenses are really expensive and bulky. Many of us just get as close as we can, use the longest lens we have, and then crop the image in our image-editing program. But that throws away pixels, reducing image quality and the size at which we can print the image. There's another way, which provides you with supertelephoto focal lengths, but without the same cost or bulk: the teleconverter (or tele-extender).
A teleconverter is a small tube containing glass elements that fits between your camera body and tele lens, and it increases the focal length. There are 1.4x, 1.7x, 2x and even 3x converters, each increasing the focal length of the lens to which it's attached by the stated amount. The big advantage is that a teleconverter costs a small fraction of what a lens 1.4, 1.7, 2 or 3 times the focal length of your longest lens costs. Attach a $150 to $500 2x converter to a 300mm lens, and you have a 600mm lens.
Teleconverters As Macro Tools
Using a telephoto/teleconverter combination like this AF-S VR NIKKOR 200mm ƒ/2G IF-ED and AF-S Teleconverter TC-20E III gives you great macro options.
Since they increase the focal length without changing the lens' minimum focusing distance, teleconverters are great tools for photographers who want to photograph insects and flowers. Add a 2x converter to a 300mm ƒ/4 lens that focuses down to five feet (0.25x magnification), and you get a 600mm ƒ/8 lens that focuses down to five feet (0.5x magnification—half life-size—from five feet away).
The light loss isn't that big a deal for close-up work because you're stopping down to increase the limited depth of field at close focusing distances. Due to the high magnification compared to work at normal shooting distances, it's best to work from a tripod, and either use flash or a higher ISO setting to get a faster shutter speed to minimize blur due to camera shake. The flash unit's very brief duration at close range also serves to minimize both camera shake and subject-motion blur.
Teleconverter Vs. Extension Tube
Kenko DG extension tubes. Extension tubes are spacers without optics in them.
The primary difference between a teleconverter and an extension tube is that the converter contains glass elements and the extension tube is just a spacer that moves the lens away from the image plane. The elements in the converter actually increase the focal length of the lens to which it's attached. The extension tube doesn't optically increase the focal length; by moving the lens farther from the image plane, it allows you to focus closer than would be possible with the lens alone, and that increases the obtainable magnification. Like converters, extension tubes reduce the amount of light transmitted to the film/image sensor; the longer the extension tube, the greater the magnification and greater the light loss. As with teleconverters, built-in TTL metering automatically handles the light loss, but you have to take it into account if using an external meter.