2x teleconverter (also known as a tele-extender) will multiply the focal length of the existing lens by 2x, and will cut back the amount of light by 2 f/stops. For example, using a 2x teleconverter with a 300mm lens will effectively increase the focal length to 600mm. A lens set to f/2.8 will now be f/5.6.
Teleconverters are not recommended for lenses wider than 50mm, e.g. 28-105mm. However, full AF operation is only possible with lenses with open aperture of f/2.8 or brighter. Please be aware that AF will work properly only if there is enough light and contrast on the subject to activate the camera's AF sensors. Manual focusing is recommended when using lenses with smaller open f-stop value than those given above.