Sigma has released a press release about the compatibility of some of their lenses with the EOS-1D X Mark II
"We have found that some Sigma interchangeable lenses for Canon are not fully compatible with EOS-1D X Mark II. When certain lenses are attached to this camera, exposure of the image may not be accurate.
"When the lenses listed below are used and either Evaluative Metering or Centre-weighted Average Metering is selected, the image could show some underexposure.
• Sigma 20mm F1.4 DG HSM | Art
• Sigma 35mm F1.4 DG HSM | Art
• Sigma 85mm F1.4 EX DG HSM
We are currently working on the firmware update for respective lenses, and the release dates will be announced later.
Usage Notice for customers who are using EOS mount Sigma lenses on an EOS-1D X Mark II:
• MO (Manual Override) function is unavailable with this camera (Full-time Manual function can be offered).
• When using a Sigma interchangeable lens for EOS, setting the corrections to [Disable] is recommended, as “Lens Correction” functions of the camera, such as Peripheral illumination correction, Chromatic aberration correction, Diffraction correction and Distortion correction, are not supported. If those functions are activated, the performance of lenses may not be accurate. This is not only for EOS-1DX Mark II, but also other camera bodies."
• MO (Manual Override) function is unavailable with this camera (Full-time Manual function can be offered).
• When using a Sigma interchangeable lens for EOS, setting the corrections to [Disable] is recommended, as “Lens Correction” functions of the camera, such as Peripheral illumination correction, Chromatic aberration correction, Diffraction correction and Distortion correction, are not supported. If those functions are activated, the performance of lenses may not be accurate. This is not only for EOS-1DX Mark II, but also other camera bodies."