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for clarification, 70-300L does not report the correct exif with the Kenko extender. it does so with the 100-400L. autofocus works, but its very jittery, and probably not good for the usm motor.
Oh well sounds like it will be impossible to make the 70-300L plus extended AF well no matter what TC .
Wish there was an option to force a toggle on to toss it into f/8 mode and manually inform the camera that a TC was being used with a lens without the extra pins.
Oh well. What a shame, because the IQ quality is perfectly solid with the combo. I suppose it could be sort of be used as is on very stationary targets if you forced it refocus a bunch of times and fired away and picked out the one frame where the focus settled in OK. The way it oscillates the AF it make me worried it strains the AF system though as you mention too so maybe liveview 10x is the only way to go, tricky though.
Likewise the AF also fails when using extension tubes with the 70-300L to unfortunately.