Canon has released firmware version 1.1.2 for the Canon EF 400mm f/2.8L IS III USM Lens
and the Canon EF 600mm f/4L IS III USM Lens
This is a very-welcomed update.
Download the new firmware here:
EF 400mm f/2.8L IS III
EF 600mm f/4L IS III
Firmware Version 1.1.2 incorporates the following enhancements:
1. Increases the manual focus speed when the manual focus speed switch is set to 1 or 2. *The speed when the switch is set to 3 (slowest speed) will be unchanged.
2. Improves the image stabilization effect during handheld shooting with a slow shutter speed when used with EOS 5D Mark IV, EOS 6D, or EOS R.
*Enables a full-time manual focus function during Servo AF when used with the EOS-1D X Mark II. The lens must be sent to the Factory Service Center in order for this funcion to be enabled.
If the lens' firmware is already Version 1.1.2, it is not necessary to update the firmware.
The lens' firmware version can be checked from the camera's menu.