From Canon :
Thank you very much for using Canon products.
We would like to inform you of the firmware for the interchangeable lens “EF 400mm F2.8L IS III USM” and “EF 600 mm F4L IS III USM” for SLR cameras, which will be released soon.
- EF 400mm f/2.8L IS III USM
- EF 600mm f/4L IS III USM
The following fixes will be incorporated into the firmware (Version 1.0.8), which will be released soon.
Corrects the phenomenon where the exposure may flicker slightly if recording a movie with the camera shooting mode set to M or Av in combination with select cameras*
* EOS-1D X Mark II, EOS-1D X, EOS-1D C, EOS-1D Mark IV, EOS 5D Mark IV (Ver. 1.0.2 or later), EOS 5D Mark III, EOS 5D Mark II, EOS 5Ds, EOS 5Ds R, EOS 7D Mark II, EOS 7D, EOS 6D, EOS 80D, EOS 60D, EOS 60D, EOS 60Da, EOS 8000D, EOS Kiss X 90, EOS Kiss X 90, EOS Kiss X 80, EOS Kiss X 50, EOS Kiss X 8i, EOS Kiss X X7i, EOS Kiss X7, EOS Kiss X6i, EOS Kiss X5, EOS Kiss X4, EOS M, EOS M2
How to avoid this phenomenon
When shooting movies, set the camera's shooting mode to P or Tv mode.
When using the lens alone or with the EXTENDER EF 2x, set the exposure setting step to 1/2 or 1/1 with the camera's custom function, even if the camera shooting mode is M or Av.