"Auto Levels corrects images with an overall color cast. To do this, it analyzes each channel individually and maps the pixels with the lowest values to 0 and the highest values to 255. The new Auto Contrast command, on the other hand, adjusts poor image contrast based on pixel luminosity, which it calculates based on a weighted average of the RGB values (remember, you asked!). It disregards the first 0.5 percent of the range of white and black pixels to ensure that it's getting representative image areas, and maps the lightest pixels in the clipped range to white and the darkest pixels to black. Highlights then look lighter and shadows darker, for finer overall image contrast."