Quote Originally Posted by piiooo


Did you check the entire range of the focal length? The 70-300's widest F-stops are 4.0 at 70mm and 5.6 closer to 300mm. How did the lens +TC autofocus at 300mm?

The new 70-300 L is not listed as being compatable with the new III series extender.


http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=3508


I wonder why? And I wonder if it is compatable with the older ones. I am not sure I would want to use a third party extender if Canon's won't work with them. What if there is a strctural issue that would damage the lens?