Of course. Fooling the camera that there's no teleconverter when there is doesn't alter the fact that a 300/4 + 2x = 600/8 and at f/8 there's just not enough light hitting the system for accurate AF.
It's partly about light, but it's more about angle of incidence. The autofocusing system is based on the rangefinder principle and at f/8, the light rays are just too perpendicular for it to accurately focus.
You found this in old manual focus cameras when the split-image rangefinder prism would turn dark with f/5.6 or darker lenses. Of course, in those days it was rare to have really dark lenses because we didn't have zooms or tele-extenders or other fancy stuff.
Karen
-- Karen Nakamura http://www.photoethnography.com/ClassicCameras/ * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
