Teleconverters and speed boosters are two common accessories that can make your lenses more versatile and even save you from having to buy a second lens. This excellent video will show you how they work as well as their pros and cons.
Coming to you from Kamil Pekala, this great video will show you how teleconverters and speedboosters work as well as their pros and cons. You can think of teleconverters and speed boosters as opposite devices. A teleconverter increases a lens' effective focal length and decreases its maximum aperture (for example, a 1.4x teleconverter will turn a 400mm f/4 lens into a 560mm f/5.6 lens), whereas a speed booster decreases a lens' effective focal length and increases its maximum aperture (thus the "speed booster" name). These can be quite useful devices; for example, a wildlife of sports photographer might carry a teleconverter to avoid having to carry a second large lens or because it would simply be too expensive to justify purchasing one. But there are downsides: besides the decrease in maximum aperture, they tend to cause a loss of sharpness, particularly 2x teleconverters, as well as decreased autofocus performance. Check out the video above for the full rundown on both devices.