Auto Lighting Optimizer (ALO) was introduced as a feature on the EOS 450D in 2008 and is included on all Canon EOS DSLR and mirrorless cameras since. The aim of ALO is to improve the look of your images through in-camera processing.

ALO is designed to reduce excessive contrast by selectively adjusting areas of the image. In this way it differs from exposure compensation, which can be used to brighten or darken the whole image. ALO is especially useful when you're using flash or shooting backlit subjects because it can detect faces in the frame and brighten those areas to achieve a more evenly-lit result and reveal more detail in shadowed areas.

ALO analyses the contrast in captured images and applies a tone curve adjustment to modify both the shadows and highlights in order to minimise loss of detail in high-contrast situations. There are three strength settings to choose from: Low, Standard and Strong. Any of the ALO settings can be used at any ISO setting. It's also possible to switch off the ALO system.

Even if you set a strength value, there's a user-selectable option to automatically disable ALO when you are shooting in Manual (and in some cases Bulb) exposure mode. This prevents the ALO system from brightening areas that you have deliberately made dark through your choice of manual exposure settings. If you choose not to enable this automatic option, you can always decide whether to apply ALO or not, whatever the exposure mode.

In other exposure modes, if you have selected exposure compensation or flash exposure compensation settings in order to make the image darker, ALO may counteract these, with the result that the image comes out brighter than you intended. In this case, either reduce the ALO strength or set it to Disable.

In some cases, noise may become more visible in the areas brightened by ALO, and the apparent resolution of the image may change. Because it reduces contrast, it can make a scene look washed-out.

Like Picture Style and white balance, ALO is a processing effect that is applied to JPEG images before they are saved to your memory card. It is not applied to RAW files, but if you use Canon's Digital Photo Professional (DPP) software, ALO can be applied during post-processing to RAW images.

Although ALO was initially found on consumer-level cameras, it is worth experimenting with for professional applications as well. If you are working to deadline, for example, it can help reduce the time needed in post-production before sending images to a client or picture desk.