Curves allow you to adjust the tonal range of your composite. Once you choose to add a curves adjustment layer, you can see the Curves panel under Layer Properties in the taskbar. In the Curves panel, you can see a histogram — a graphical illustration of pixels at every color intensity level in your image. Initially, the image’s tonality is represented as a straight diagonal line in the histogram. The horizontal axis of the graph represents the input levels (original values) and the vertical axis represents the output levels (adjusted values). On applying adjustments to an RGB image, the upper-right area of the graph represents the highlights, the middle area represents the mid-tones, and the lower-left area represents the shadows.
Under Adjustments, you can select any of the adjustment options — Brightness/Contrast, Black and white, Color balance, Curves, Exposure, Hue/Saturation, Levels, and Vibrance.
Choose Curves to apply curves adjustments.
Under Curves in the Layer Properties panel, tap anywhere on the straight diagonal line in the histogram to add nodes. As you add nodes to the diagonal line and move them, the shape of the curve changes, reflecting your adjustments. The steeper sections of the curve represent areas of higher contrast while flatter sections represent areas of lower contrast.
- Moving a node in the top portion of the curve adjusts the highlights. Moving a node in the center of the curve adjusts the midtones, and moving a node in the bottom section of the curve adjusts the shadows.
- To darken highlights, move a node near the top of the curve downward. To lighten the shadows, move a node near the bottom of the curve upward.
- Moving a node either down or toward the right maps the Input value to a lower Output value, and your composite darkens. Moving a node either up or to the left maps a lower Input value to a higher Output value, and the composite lightens.
Instead of adding a node, you can also manually enter an Input value and required Output value to apply a specific adjustment to your photoshop document.
By default, the curve adjustment layer affects all layers below it in the Layers panel. To apply your curves adjustments only to the layer placed directly below the curves adjustment layer in the layer stack, you can clip your curves adjustment layer to the layer you wish. To learn how to apply clipped curves adjustments, see Work with adjustment layers.