Auto Lighting Optimizer

Auto Lighting Optimizer (ALO) has been included on the EOS-1D Mark IV (it’s also on the 50D, 5D Mark II and 7D) to help to produce good looking images straight from the camera and reduce the time from shooting to sending. It works by evening out contrast and is especially useful when shooting with flash or with backlit portraits. It will also help improve images shot in low contrast conditions. There are three settings - Standard, Low and Strong - and disable. You should try each of them in a controlled environment to see which works best for you. Alternatively, you can use Canon’s Digital Photo Professional programme to turn ALO on and off to see the effect on a variety of images before making a choice.
Camera settings
EOS 5D Mark II
EOS-1D Mark IV