More on Amazon.com : http://www.amazon.com/gp/product/B000JSZ41S?tag=thefatkidin06-20
Opteka High Definition 2X Telephoto Converter for Opteka 650-1300mm 500mm and 800mm SLR Lenses
Doubles your zoom on the Opteka 650-1300mm 500mm and 420-800mm Lenses
Works ONLY with compatible T-mount Lenses
High Definition Glass
