The Canon Extender can be used to increase (double) the focal length of your telephoto lens, with minimal loss of lens performance or image quality. Note that using the Extender decreases your aperture by two stops: for example a lens which is f/2.8 becomes f/5.6 when the 2x Extender is used.
If you hire an Extender, please ensure it is compatible with the lens you want to use it with. We can't accept any responsibility if it isn't. If you're not sure whether your lens is compatible, please ask. We don't bite.