From Sigma:
Thank you for purchasing and using our products.
We would like to announce the availability of new firmware and support for the applicable lenses regarding the phenomenon that exposure of the image may not be accurate, which was announced on May 27th, 2016. This happens when some SIGMA interchangeable lenses for CANON are used on Canon EOS 1DX Mark II.
If you own the following applicable products, please refer to the information below and update the lens firmware accordingly.
Benefit of this firmware update
The lens firmware update corrects the phenomenon of some underexposure when the lenses listed below are used and either “Evaluative Metering” or “Center-weighted Average Metering” is selected in Metering Mode of the camera.
Applicable products
SIGMA 20mm F1.4 DG HSM | Art – Canon mount
SIGMA 35mm F1.4 DG HSM | Art – Canon mount
SIGMA 85mm F1.4 EX DG HSM – Canon mount
For customers who own the applicable products listed above, the lens firmware update will be provided free of charge. Please contact your nearest authorized subsidiary/distributors of SIGMA.
For customers who own the SIGMA USB DOCK, and either the SIGMA 20mm F1.4 DG HSM | Art Canon or the SIGMA 35mm F1.4 DG HSM | Art Canon, please update the lens firmware using SIGMA Optimization Pro.
We appreciate your continued support for our company and products.