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 m