In theory, teleconverters are a simple, affordable fix for increasing the focal length of any lens. But in reality, teleconverters are not the saviors they are made out to be. They aren't some magic device that can cheat physics and come with some serious drawbacks.  In addition to increasing the apparent focal length, they also reduce the amount of light entering the camera. A 1.4x teleconverter loses 1-stop of light, and a 2x teleconverter loses 2-stops of light. You can mitigate the loss of light by increasing the ISO to compensate, but depending on the lens, using a teleconverter can make autofocus slower or stop working altogether.  The biggest problem with using teleconverters is the loss of sharpness inherent in using them. Even the most expensive lenses suffer from image degradation with the use of teleconverters, and the less expensive the lens, the more image degradation it will experience. This isn't meant to dissuade you from using them, but it is meant to remind you of their effects on image quality and camera performance.