Digital camera features: Auto Lighting Optimizer
Auto Lighting Optimizer (ALO) was introduced as a feature on the EOS 450D DSLR in 2008. It is found on all current Canon EOS DSLR cameras, from the entry-level EOS 1100D to the professional EOS-1D X, with the exception of the EOS-1Ds Mark III. The aim of ALO is to provide in-camera processing that will improve the look of your images.
ALO is designed to even out contrast by selectively adjusting areas of the image. It is especially useful when using flash or with backlit subjects as it can detect faces in the frame and brighten those areas to achieve a better result.
ALO analyses contrast in captured images and modifies both shadows and highlights via tone curve adjustments to minimise loss of detail in contrasty light conditions. There are three levels of setting strength as well as the ability to turn the setting off (known as Disable). The levels are Standard, Low and Strong and these can be used at any ISO speed setting.

The Auto Lighting Optimizer levels as displayed on a EOS DSLR's Custom Function menu screen.
If you make use of Canon's Digital Photo Professional workflow software, ALO can be applied to RAW images taken on compatible cameras during the post-processing workflow.
Although ALO is a feature initially found on consumer level products, it is worth experimenting with for professional applications as well. If you are working to deadline it can help reduce the time needed in post-production before sending images to a client or picture desk.

In this backlit image, with Auto Lighting Optimizer, disabled the face appears too dark.

With Auto Lighting Optimizer enabled the exposure has been balanced out and the subject’s face appears brighter.