> It's a 6 element TC optimized for shorter focal lengths that > _also_ has a built-in helical extension mechanism.
I just dug out the owners manual for the Vivitar 2X Macro-Focusing TC, and it says: "The Vivitar Macro Focusing Teleconverter is designed to work with a variety of high quality lenses: 50mm and other fixed focal length lenses from 28mm to 400mm, plus zoom lenses - including wide to tele and telephoto zooms." This is actually a more generous range of focal lengths than Pentax recommends for the 6-element T6-2X (135mm to 300mm). Fred
