Autofocus on the 6D does not function below f/5.6. This means AF stops working if you use a 1.4x extender with a lens with a maximum aperture darker than f/4, or if you use a 2x extender with a lens with a maximum aperture darker than f/2.8, because they cause you to lose 1 or 2 stops respectively.
Last edited by javabytes; Aug 4, 2015 at 9:02 pm