Placed between the camera body and lens, a teleconverter contains a set of optics that will effectively increase the focal length of a lens by a factor of 1.4 times. Pro-optic converters are made with high quality multi coated optics and designed to electronically operate with the camera contacts.. Designed specifically to be used with prime or zoom lenses of 100mm or above. 1.4 will increase a 300mm into 420mm and will decrease the lens with 1 f/stop Focusing manually is recommended when using a lens with a f/stop of f4 or slower. Longer exposures will require a tripod for stabilization.