Auto Lighting Optimizer and Highlight Tone Priority - Test on Canon 550D and 17-40 F/4L

Canon DLSRs of recent years have in-camera features that are supposed to extend Dynamic Range. One feature - Highlight Tone Priority (HTP) - aims to retain details and prevent clipping of highlights (bright areas). Another feature - Auto Lighting Optimizer (ALO) - aims to lift details from shadows (darker areas). ALO is deemed useful for high contrast scenes, eg, backlit subjects; HTP is deemed useful for high-key shots. Both are used on jpegs.

For the Canon 550D, HTP is enabled or disabled within the menu (Cn Fn 6), while ALO has four settings (Off, Low, Standard, Strong). ALO is disabled if HTP is turned on.

I did a few test shots using the EF 17-40 mm F4/L lens. The images below were taken a few seconds of each other and have not been edited except for resizing. Aperture f/5.6 and shutter speed at 640 and ISO 100, except for the image with HTP on at shutter speed of 1250 and ISO 200.

The effects of HTP and ALO are not subtle. Which setting to use is a matter of preference.

Colors with HTP on seem more saturated, but it does make the overall image darker. I can see that it could be good way to prevent blown out highlights when needed, but I do notice some noise in the sky.

For ALO, the effect is quite pronounced specially at the Strong setting. Here are 100% cropped images from the center of the frame.



The 550D does this processing as it converts the image to jpeg.

One can achieve the same effect with post-processing. I applied the simple "curves" tool in PaintNet to slightly brighten the ALO Low image, and to slightly darken the ALO Strong image; here they are  with the ALO Standard sample.

My personal preference is to turn off HTP and keep the ALO Low setting as default.

Of course, one can always shoot Raw and do all of this in post-processing, but the HTP and ALO features are shortcuts for jpeg shooters, which could be handy for casual shots such as for holidays and family events.