“When you’re shooting with long lenses, even if you’re shooting a close-up, you feel the air, the distance between the camera and the subject.”—Emmanuel Lubezki
Teleconverters are small optical devices that fit between your camera body and lens producing greater magnification and increasing the lens’ focal length. A 2X extender doubles the focal length of the lens but there’s no free lunch. It also gobbles up light to the tune of two f/stops so the f/1.8 lens you attach to a 2X extender is now f/4. That’s not a bad trade but if you put it on a slower lens it many cause autofocus problems.
Many AF systems require a maximum lens aperture of f/8 or less in order to function effectively. So do the math before attaching a 2X extender to a smaller aperture zoom lens and be sure to read your camera manual, as painful as that may be for some of us.
Having a teleconverter is also useful for macro work because it magnifies the size of small objects, such as the above butterfly that was photographed using a Canon EF 50mm f/2.5 Compact Macro lens and 2X teleconverter. Exposure was 1/25 sec at f/14 and ISO 400 with a Canon MR-14 EX II ringlight as a light source.
There are times when you don’t need a 2x lens conversion and other times when you just can’t afford two stops of light loss. That’s when you may need something like a 1.4x teleconverter that turns a 300mm lens into 420mm but only decreases the maximum lens aperture by one f/stop.
Having two teleconverters in your camera bags triples the number of focal lengths available and the prices ($399 for Sigma 2X models) for these devices represent an optical bargain.