The tamron tc x20 20x teleconverter for canon dslrs mounts between the camera and lens to increase the focal length by a factor of 20x designed exclusively for select tamron lenses when used with the sp 150 600mm f5 63 di vc usd lens the teleconverter will give the apparent focal length of 300 1200mm the tc x20 is the perfect tool for gaining that extra bit of reach without adding lots of additional bulk and weight to your kit bag