In this example, we’re using a stunning backlit photo of a lion. However, the image feels flat and lacks contrast due to how the subject is lit. We’ll use the Luminance channel in Levels to correct the tone and contrast issues.
Open an image in Single Photo View, click the edit icon
, then twirl down the arrow 
next to Levels.Click the L (luminance) button, then move the white point slider slowly to the left. This affects the image’s white point and allows you to reassign where white should begin in the image.
Move the black point slider slowly to the right. The more you move the slider to the right, the more contrast is introduced into the image.
Move the midtones slider. Moving the slider to the left will add more light, and moving it to the left will subtract light.