Basically, the higher and more horizontal the lines are (the closer to the top horizontal line 1), the better the image quality is.


According to the charts on the Canon website, the 500mm II plus 1.4X III looks to be as good as the original 600mm on it's own.


I would have to expect the AF to be just as good with the improved optics of the glass, not to mention the improved 4 stop (Mode 3) IS.