One of the more obscure camera questions I occasionally field on my workshops from participants who shoot Canon is ‘what is Canons Auto Lighting Optimiser and should I be using it?’ I received this exact question this morning via email from a Canon user who just received their new camera and wanted to understand how this feature works.
First, some history on the Auto Lighting Optimiser (ALO). ALO was originally introduced as a feature on the EOS 450D DSLR in 2008. It has since found its way onto all current Canon EOS DSLR cameras, from Canon’s most basic entry-level DSLR to the professional EOS-1D X MKII (it was never included in the 1DS MKIII).ALO is designed to even out contrast by selectively adjusting areas of the image. It is by definition most 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. In a nutshell, the aim of ALO is to provide in-camera processing that will improve the look of your images. Those of you paying attention will have already figured out that it applies to jpeg files because it is working on in camera processing.
ALO analyses contrast in captured images and modifies both shadows and highlights via tone curve adjustments to minimise loss of detail in high contrast light conditions. There are three levels of setting strength in the current EOS 1DX MKII (low, medium and high) as well as the ability to turn the setting off. The levels can be used at any ISO speed setting.
The first thing you should know about ALO is where it is located in your cameras menu. ALO is found in the jpeg quality menu. This tells us that this setting applies to jpegs (not RAW files). If you shoot RAW then you can pretty much ignore ALO; with the caveat that it can be applied in post-production if you implement Canon’s Digital Photo Professional Software. If you are using Canon’s Digital Photo Professional workflow software (I honestly don’t know anyone who does), ALO can be applied to RAW images taken on compatible cameras during the post-processing workflow. This cannot be accomplished in Adobe Lightroom. Is this worth doing? Personally, I don’t think the benefit is worth the extra step (I would rather use a filter to tame the scene contrast in camera), but your mileage might vary.
When you select a Quality setting that results in a JPEG image file (have I mentioned you really should be shooting RAW?), ALO tries to enhance your photo while it’s processing the picture in camera. Unlike Highlight Tone Priority, which concentrates on preserving highlight detail only, Auto Lighting Optimizer adjusts both shadows and highlights to try and improve the final image tonality. This ‘could’ be useful if you are shooting jpegs in high contrast lighting and have no intention of doing post production work at a later date.
To sum up: If you shoot RAW (as I do) then you can ignore the ALO setting and just leave it off. If you insist on shooting jpeg then you should experiment with ALO to see what benefit it can yield to your jpeg files in certain high contrast lighting situations.