Canon EOS-1D X Mark III Af Setting Manualbook page 71

f/8 compatible AF
The EOS-1D X Mark III offers f/8 compatible AF* over the
entire AF area with the optical viewfinder. Therefore, using
an Extender EF 2x III attached to a lens that has a maximum
aperture of f/4, or an Extender EF 1.4x III attached to a
lens that has a maximum aperture of f/5.6 now means,
AF is possible. For example, when an Extender EF 2x III is
used attached to the EF 600mm f/4L IS III USM, AF shooting
is possible with a focal length of 1200mm.
* Depending on the lens and conditions, there will be times when support
is not possible.
EF 600mm F4L IS III USM
Extender EF 2× III
Examples of lens configurations that can be used
with AF
When using Extender EF 2x III
EF 300mm F4L IS USM
EF 500mm F4L IS II USM
EF 600mm F4L IS III USM
EF 70-200mm F4L IS II USM
When using Extender EF 1.4x III
EF 800mm F5.6L IS USM
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