How It Works
Using RGB Level sliders to trim and adjust available information, as noted in the previous sets of steps, helps to increase tonal range in each of the primary colors. It does this by taking available image information and redistributing the tonal mapping over a broader range.
The tone for each color runs from black to white and displays a full range, instead of tones running only part of the potential range (e.g. from black to a light gray, as is the case in this example). This change increases overall image contrast and improves the depth of the color. This simple maneuver also corrects many color casts because it neutralizes the effect of many 'natural' color shifts (color of lighting, color of filters, many types of fading, etc.).
Moving any of the sliders to the left of its original position will effectively lighten the image overall; moving sliders to the right will darken the image. This is because as the value of the slider increases, the amount of image information that falls into the brighter range also increases (essentially more levels of tone fall into the right hand, or bright side, of the slider and histogram graph).
After using the Levels to enhance color and tone to their full potential range, Color Balance allows you to adjust the influence of colors and color interplay. As the technique is based only on visual evaluation, your monitor must be calibrated.