Teleconverters aka teleextenders 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.
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 need something like a 1.4x teleconverter that will turn a 300mm lens into 420mm but only decrease the maximum lens aperture by one f/stop.
Having two teleconverters in your camera bags triples the number of focal lengths available and the affordable prices (under $250 each for Tamron’s SP models) for these devices represent an optical bargain.