Making Tonal Adjustments to an Image

You can adjust the contrast selectively along the spectrum of dark and light shades.

Many digital images, particularly those produced on a scanner, display less detail in darker or shadowed regions of the image. Adjusting brightness and contrast for the entire image can improve the level of detail in the darker regions, but usually does so at the expense of the midtones and lighter colors. Controls on the Tonal Adjustment tab of the Histogram dialog box can adjust the contrast more selectively. You can, for example, increase the contrast in the darker tones, while maintaining the current level of contrast among midtones and lighter colors.

Three types of contrast curve are available:

• A Gamma correction curve is exponential. You can either specify the single exponent (gamma) value or use an interactive slider to determine the gamma value.
• A Fitted curve is drawn smoothly through points you specify. You can add or remove points from the curve, and drag them to modify the shape of the curve.
• A Piecewise Linear curve is constructed of straight-line segments between points you specify.

You can use a grid as a visual reference for constructing the curve. You can turn the grid on and off by right-clicking in the curve window and selecting Grid on the shortcut menu.

Two sliders under the curve control the lower and upper cut-off points for the curves. Input values below the lower cutoff point are displayed as 0, while input values above the upper cutoff point are displayed as 255. Curves can be saved and imported as Gamma Point List (*.gpl) files.

Like the other histogram functions, when you are operating on a true color image, you can choose to manipulate a single color channel, or all three channels together.

Note: The Tonal Adjustment feature does not apply to bitonal images.

Above the curve window, the histogram window displays input and output histograms. The input histogram is a graph, shown in gray, representing the number of pixels in the original image containing each of the given colors. Similarly, the output histogram previews the number of pixels present for each of the given colors in the image after the curve is applied.

By default, the histogram window adjusts the vertical scale of the histograms such that the entry with the highest frequency extends to the top of the histogram window. You can use the vertical scale slider to effectively zoom in on the histogram. As you move the slider upward, the scale is increased so that low frequency values are more visible and high frequency values are truncated by the top of the window.