7D, Canon 1.4 vIII, 800mm auto focus inoperative

[lv2rescue]

HI folks,
I bought an 800 to use on my 1DX and then I put the 800 on my 7D and a VIII 1.4 Canon converter and I cannot get the autofocus to function. Is this an incompatibility issue I overlooked? Thanks for any feedback.

 

[Loswr]

The 7D requires an f/5.6 max aperture for phase (viewfinder) AF. The 1.4x TC costs a stop, so the 800mm f/5.6 becomes an 1120mm f/8 lens. You'd need a 5DIII or post 1DIII 1-series body (the 1D X got f/8 AF via a prior firmware update) for regular AF. Live View AF will work on your 7D.

 

[mackguyver]

Take a look at my post here for some pointers on technique - the shot in the post was shot using that same lens/extender combo on a 60D:
http://www.canonrumors.com/forum/index.php?topic=18528.0