Photoshop Color Correction - Giordan on Graphics | 2
The Levels Control is another way to modify the color in an image. Like curves, Levels also lets you change the color across the tonal range, modifying a composite image or individual color channels. The dialog features a histogram method of displaying image data that is unique and sometimes useful. It shows the full range of tonal values, from light to dark, on a linear scale. A graph rises and falls across that scale, indicating the actual number of pixels in the image that have the corresponding value. Therefore, as a general rule you would want a nice rounded curve that is highest in the mid-tones, with good representation in the highlight and shadow ends of the spectrum.
The Levels control uses highlight, midpoint, and shadow slides, which shift the image values in the histogram, lightening or darkening the image. I find this three variable approach limiting compared to the unlimited number of points I can place and manipulate on a curve. On the other hand, until I became comfortable with color numbers, Levels was my main color control. It also allows you to set white and black points, and delivers decent results for basic to intermediate changes.