Canon EOS 5D Mark III Instruction Manual page 85

AF when the maximum aperture is f/8
When an extender is attached to the lens, AF is possible even when
the maximum aperture value is larger than f/5.6 up to f/8. Selectable
AF area selection modes are the same as that of group H (p.84).
EF400mm f/5.6L USM
+ Extender EF1.4x
EF500mm f/4.5L USM
+ Extender EF1.4x*
EF800mm f/5.6L IS USM
+ Extender EF1.4x
EF1200mm f/5.6L USM
+ Extender EF1.4x*
EF300mm f/4L USM
+ Extender EF2x
EF300mm f/4L IS USM
+ Extender EF2x
EF400mm f/4 DO IS USM
+ Extender EF2x
If the maximum aperture is smaller than f/5.6 (the maximum aperture
value is larger than f/5.6 up to f/8), focus may not be achieved with AF
when shooting low-contrast or low-light subjects.
When Extender EF2x is attached to the EF180mm f/3.5L Macro USM
lens, AF is not possible.
If the maximum aperture is smaller than f/8 (the maximum aperture value
exceeds f/8), AF is not possible during viewfinder shooting. Also, AF is
not possible with f during Live View shooting and movie shooting.
EF400mm f/4 DO IS II USM
+ Extender EF2x
EF500mm f/4L IS USM
+ Extender EF2x
EF500mm f/4L IS II USM
+ Extender EF2x
EF600mm f/4L USM
+ Extender EF2x*
EF600mm f/4L IS USM
+ Extender EF2x
EF600mm f/4L IS II USM
+ Extender EF2x
Lenses and Usable AF Points
EF70-200mm f/4L USM
+ Extender EF2x
EF70-200mm f/4L IS USM
+ Extender EF2x
EF100-400mm f/4.5-5.6L IS USM
+ Extender EF1.4x
EF200-400mm f/4L IS USM
Extender 1.4x:
With built-in Ext. 1.4x + Ext. EF1.4x
EF200-400mm f/4L IS USM
Extender 1.4x + Extender EF2x
85