Working with Professional Color Correctors in Photoshop

The simple color correctors I’ve shown you so far in this chapter usually aren’t enough to provide thorough color correction if you have a really problematic image on your hands. Fortunately, Photoshop has the kind of professional tools needed to make sophisticated color corrections required for reproduction. You don’t have to be a pro to use the Levels or Curves commands, nor to work with the Hue/Saturation controls. But you’ll feel like one after you master these powerful tools.

Leveling for better contrast
I described the automatic variation of the Levels command earlier in this chapter. If you want to adjust tonal values of images (the brightness or darkness of tones) or correct colors (the relationship between the colors), this is the tool for you. The Levels command is a much more sophisticated tool than the Brightness/Contrast control because you can work with individual tones, brightening or darkening individual tones as you want, and you have a great deal more information to help you make your choices. Open the Levels dialog box by pressing Ctrl+L (Ô+L on the Mac) or selecting Image-->Adjustments-->Levels. The graph shown in the center of the dialog box is a histogram. Remember, you can use the histogram to measure the number of pixels at each of the 256 brightness levels. Each vertical line represents the number of tones at each of the brightness levels. The default histogram displays information for the entire image, but you can view separate histograms for each of the Red, Green, and Blue channels (or the Cyan, Magenta, Yellow, and Black channels if you happen to be working with a CMYK image).

The horizontal axis displays the range of these pixel values, with 0 (black) on the left and 255 (white) on the right. The vertical axis measures the number of pixels at each level. The pattern of lines in the histogram shows you when an image has a preponderance of one kind of pixels (say, bright or dark ones), as well as gaps in the distribution that represent tones that are not represented at all. 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, or no white tones at the far-right side.

One of the simplest corrections you can do is to move the black and white sliders so that they actually correspond to the pixels containing dark and light tones. Simply slide the black triangle so that it corresponds to the first true black pixels in the image (the beginning of the histogram), and then move the white triangle to align it with the lightest pixels (the end of the histogram). That ensures that Photoshop doesn’t waste tones by allocating them to areas of the image that actually have no image detail.

The Levels dialog box has an Auto button, which operates similarly to the Auto Levels command, except that you can see exactly what happens. When you click the Auto button, 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.

Setting black and white points manually
For more control, you can use the eyedropper tools in the Levels dialog box to set the black and white points. Just follow these steps:
1. Open an image and choose Image-->Adjustments-->Levels.
Make sure you have the Info palette open and that the HSB and RGB color models are displayed. Remember that you can also apply certain adjustments, such as Levels, via an adjustment layer rather than directly to the image itself. Adjustment layers provide more editing flexibility if you later decide you need to tweak the adjustment.
2. Select the White Eyedropper tool and drag it around the image while watching the Info palette.
3. Look for the lightest white in the image, which may be anywhere from 90 to 100%. Select that point by clicking.
4. Use the Black Eyedropper tool to select the darkest black in the image.
The combination of these two choices redistributes the pixels from pure white to pure black. You can also reset the white and black points by moving the position of the white and black triangles on the input sliders (upper scale). Or, you can enter numbers in the Input Levels boxes. The three boxes represent the black, gray, and white triangles, respectively. Use the numbers 0 to 255 in the white and black boxes.
5. Use the Gray Eyedropper tool to remove any color casts. Select a neutral gray portion of your image, one in which the Info palette shows equal values of red, green, and blue.
Note that the Gray Eyedropper tool is not available when working on grayscale images. Although you generally make changes to the entire document by using the RGB channel, you can apply changes to any one of an image’s component color channels by selecting the specific channel with the Channel pop-up menu.
6. Adjust the output sliders.
Moving the black triangle to the right reduces the contrast in the shadows and lightens the image. Moving the white triangle to the left reduces the contrast in the highlights and darkens the image.
7. Adjust the midtones with the gray triangle slider.
The values you’re adjusting are called the gamma values. Dragging this triangle to the left lightens the midtones. Dragging it to the right darkens the midtones while leaving the highlights and shadows alone. You can also move the gray triangle by entering numbers from 9.99 to 0.1 in the center option box. The default value, 1.0, lies exactly in the middle of the range. If you’re working with a series of similar images (such as a bunch of video captures), you can save the settings to reuse them later.
8. Click the Save button to store your settings.
(Just click the Load button to retrieve them.) This saves the settings, but doesn’t apply them.
9. Click OK to apply your settings and exit the dialog box.