The Auto Level transform operator is similar to Auto Contrast , except that whereas Auto Contrast sets contrast by reckoning pixel intensity considering all channels taken together, Auto Level sets levels within each channel separately.
Auto Level will often produce better contrast than either Contrast or Auto Contrast. However, because it adjusts intensities in each channel separately to produce the overall effect it can make minor color shifts. These minor color shifts will often enhance the sense of reality and vividness of an image.
If take our standard schloss image above and apply Auto Contrast we get the image below.
If we start with the schloss image and apply Auto Level we get the image below. Compare the difference in appearance of trees in the image above with those in the image below. The Auto Level image provides slightly greener trees. There is also a greater sense of color differentiation in the yellow crenellations of the uppermost towers.
Compared to the original image the Auto Level image has slightly different color tones but in a productive way that yields a more vivid image. In other images, the color shift may be objectionable. For such images Auto Contrast may be a better choice.