Placed between the camera body and lens, a teleconverter contains a set of optics that will effectively increase the focal length of any lens with which it is used.
The Kenko Teleplus HD DGX 1.4X converter has the effect of multiplying the focal length of your lens by 1.4X, turning a 300mm lens into an equivalent 420mm.
The HD DGX 1.4X converters have genuine Gate Array IC (Integrated Circuitry). This means the converter's unique circuitry maintains signal integrity between the camera body and lens.