The Curves dialog box, shown below, offers more flexible control over tonal quality than Levels. The graph shows a mapping of tonal values from input to output. Input values are represented on the bottom of the graph, and output values are on the left side of the graph. The following curve represents a simple linear mapping. Each value in the input maps to an identical value on the output.
The slope of the straight-line portion of the curve is known as gamma. A gamma of one, illustrated above, indicates no change in contrast. Gamma values greater than one, or an increase in slope, indicate an increase in contrast. Gamma values less than one, or a decrease in slope, indicate a decrease in contrast. To change the curve, click on the curve to add a point. Drag the point to alter the shape of the curve. To delete a point, drag the point off the graph.
To lighten an image, drag the center point to the left. This causes more pixels to be mapped to the upper half of the output, and results in a lighter image. This adjustment corresponds to moving the gray slider to the left in Levels. Shadows have increased contrast (gamma > 1), and highlights have decreased contrast (gamma < 1).
To darken an image, drag the center point to the right. This causes more pixels to be mapped to the lower half of the output, and results in a darker image. This adjustment corresponds to moving the gray slider to the right in Levels. Shadows have decreased contrast (gamma < 1), and highlights have increased contrast (gamma > 1).
To increase contrast drag the endpoints of the curve horizontally toward the center. This increases gamma (slope of the curve) and maps a small range of tonal values to the output. The result is an increase in contrast and corresponds to moving the black and white input sliders toward the center in Levels. This adjustment may truncate input values and result in a loss of detail in shadows and highlights.
To decrease contrast drag the endpoints of the curve vertically toward the center. This decreases gamma (slope of the curve) and maps the input values to a small output range. The result is a decrease in contrast and corresponds to moving the black and white output sliders toward the center in Levels. This adjustment reduces the dynamic range so that blacks are not black and whites are not white.
To increase contrast without losing detail, place and drag two points to make an S-shaped curve as shown below. This increases midtone contrast, while contrast is reduced in shadow and highlight areas. The net result is a perceived increase in contrast while retaining detail in the shadows and highlights. This adjustment is usually preferable to a simple straight line adjustment where shadow and highlight detail is lost.
Linear S-Shaped Curve Increase Contrast
To decrease contrast without sacrificing dynamic range, place and drag two points to make an S-shaped curve as shown below. This decreases midtone contrast, while contrast is increased in shadow and highlight areas. The net result is a perceived decrease in contrast without a loss of dynamic range. This adjustment is usually preferable to a simple straight line adjustment where deep blacks and bright highlights are lost.
Linear S-shaped Curve Decrease Contrast
You can also increase contrast using an Overlay blend. Create any adjustment layer with default settings. Yes, that's right, any adjustment layer will do. Change the blending mode to Overlay, then adjust contrast with the Opacity slider. The net effect is the same as an S-shaped curve with varying slope. I often use this technique in actions, such as the Black & White action, for a simpler interface.
