Canon has posted EOS 1D Mark IV Firmware v1.1.1
. Note: you must select your OS to get the firmware download option.
Firmware Version 1.1.1 incorporates the following change.
Optimizes the camera’s exposure accuracy when using EF 24mm f/2.8 IS USM and EF 28mm f/2.8 IS USM lenses, which are scheduled for release in June 2012.
If these lenses are used with an EOS-1D Mark IV camera that has not been updated with the new firmware, the image may be overexposed depending on the subject and shooting scene.