Another use of Curves is to perform a custom white balance. All that is required is that the photographer shoot a neutral gray object (e.g., a gray card) in the same lighting conditions as the regular shots. If shooting JPEG, the camera needs to be set to one of the preset white balance options (e.g., daylight or cloudy) for both shots (do not use auto white balance). If the image is converted from a raw file, the white balance settings in the raw converter need to be the same for both the regular and gray card images (to make it easier on yourself, I suggest that a preset white balance setting be used even with raw when you plan to use Curves to set the white balance).
Figure 1 shows an image of a ladybug that was shot in a field of green. The large amount of green caused problems for the auto white balance. There were no guarantees that even a preset white balance setting would get the color perfect. So, to insure proper white balance, a gray card was shot right after the scene was photographed (see Figure 2).
The Color Picker dialog box appears as shown in Figure 5. This Color Picker is used to set the gray values. These values will then be assigned to whatever point is clicked with the Gray Eyedropper. The values are set by entering the values directly into the R, G, and B boxes. Since a neutral point is going to be identified, all of the values should be equal. It is necessary to match these values to the gray object that was photographed. For most neutral gray objects, values of 128, 128, and 128 will be appropriate. Clicking OK closes the Color Picker.
The last step is to click the Gray Eyedropper on the gray card image as shown in Figure 6). The ladybug image is now custom white balanced.
In some cases, it may be desired to target an image for printing to a particular printer. In such a case, the problem that must be dealt with is that all printers have printing limitations. In particular, not all printers and printing presses can print smoothly from pure black to pure white. For instance, as a printer prints progressively darker tones, it lays down more and/or larger dots of ink. However, if the resolution of the printer is not high enough, a point may be reached where the printer does not have enough fine control to differentiate between the darkest tones. When this happens, all of the tones beyond that point will turn to black in the print. As an example, a specific printer may not be able to differentiate between tones with a value of eight or lower. If an image were printed without proper adjustment, the tones from zero to eight would all be black. The detail in the image between these tonal values would be lost during the printing process -- these tones would be clipped.
On the highlight side, as a printer prints progressively lighter tones, it lays down less and/or smaller dots of ink. However, if the resolution of the printer is not high enough, a point may be reached where the printer does not have enough fine control to differentiate between the lightest tones (the printer can not make the dots any smaller or fewer). When this happens, all of the tones beyond that point will turn to the white of the paper. As an example, a specific printer may not be able to differentiate between tones with a value of 247 or higher. If an image were printed without adjustment, the tones from 247 to 255 would all be white. The detail in the image between these tonal values would be lost during the printing process -- these tones would be clipped.
Curves can be used to solve this problem by targeting the shadows and highlights for a specific printer.
To target the shadow values, the lower left end of the curve is dragged upward until the desired values are indicated in the Input and Output boxes (the input value will always be 0). An alternative is to type the values directly into the boxes. Figure 9 shows Curves used to target the shadow values. In this case, the tonal value of 0 has been mapped to a tonal value of 8 in order to fit the shadows within the tonal range that the printer is capable of printing.