(for 300mm or longer lenses, except some older model FD lenses)
The Canon Extender FD 1.4X Type A is a very special accessory designed to increase the focal length of long telephoto lenses with a minimum loss of light transmission. When added to Canon telephoto or super telephoto lenses, their focal length is effectively increased 1.4X, eg: 300mm becomes 420mm, 600mm becomes 840mm, etc.