How to Get Better Contrast in Photoshop CS6 Levels Command
If you want to adjust tonal values of images in Photoshop CS6 or correct colors, the Levels command can give you better contrast and is the tool for you. It offers more control than the Auto Tone command. The Levels command is also a much more sophisticated tool than the Brightness/Contrast control because you can work with individual tones and you have a great deal more information to help you.
Open the Levels dialog box by pressing Ctrl+L (Command+L on the Mac) or choosing Image→Adjustments→Levels. The graph shown in the center of the dialog box is a histogram. You can use this dialog box, histogram and all, for evaluating and adjusting levels in the following ways:
Visually check the distribution of dark, midtone, and light values.
View separate histograms for each channel. The default histogram displays information for the entire image. To see the histogram of an individual channel, select it from the Channel pop-up. For an RGB image, you can view the Red, Green, and Blue channels. For a CMYK image, you can view the Cyan, Magenta, Yellow, and Black channels. You can view the histograms of each channel simultaneously in the Histogram panel.
Adjust the black and white points based on the histogram. The three triangles at the bottom of the histogram, in black, gray, and white, represent the shadow on the left, midtone in the middle, and highlight on the right. Even though they’re located where they are, many images have no black tones at the far-left side of the scale nor white tones at the far-right side.
One of the simplest corrections you can do is move the black and white sliders to correspond to the pixels containing dark and light tones. Simply slide the black triangle to correspond to the first true black pixels in the image, then move the white triangle to align it with the lightest pixels. That ensures Photoshop doesn’t waste tones by allocating them to areas that have no image detail.
See exactly what happens when you use the Auto Tone command. When you click the Auto button, which applies the same adjustments as the Auto Tone command, Photoshop applies its own suggested changes, resetting the white point and the black point, and redistributing the gray values of the pixels in between. Afterward, the histogram shows that the pixels fill the complete range from white to black.