The Curves tool in Photoshop is a powerful way to adjust the tonal range and contrast of your images by manipulating a graphical representation of the tones.
Essentially, the curves tool can take input tones and selectively stretch or compress them, similar to the Levels adjustment. However, where Levels offers control primarily over black, white, and midpoint, Curves provides far greater flexibility. A tonal curve, displayed as a line on a graph, is controlled using any number of anchor points—small squares you can add along the line, up to a total of 16. By dragging these points, you remap the original brightness values (input) to new brightness values (output).
Understanding the Curves Graph
The Curves adjustment uses a graph where:
- The horizontal axis represents the original pixel values (Input Tones). It ranges from 0 (pure black) on the left to 255 (pure white) on the right.
- The vertical axis represents the new, adjusted pixel values (Output Tones). It also ranges from 0 (pure black) at the bottom to 255 (pure white) at the top.
- The diagonal line initially runs from the bottom-left corner (input black to output black) to the top-right corner (input white to output white). This straight line means no adjustment is being made—each input value equals the output value.
How Anchor Points Shape Your Image
The magic of Curves lies in adding and manipulating anchor points along this diagonal line.
- Adding Points: Click anywhere on the line to add an anchor point.
- Dragging Points: Drag an anchor point up or down to change the output value for the corresponding input value.
- Dragging a point up brightens the tones in that range.
- Dragging a point down darkens the tones in that range.
- Targeting Tones: You can add points to specific areas:
- Near the bottom-left to adjust shadows.
- In the middle to adjust midtones.
- Near the top-right to adjust highlights.
By adding multiple points, you can create complex curves that selectively adjust different tonal ranges independently, giving you precise control over contrast and brightness.
Curves vs. Levels: A Quick Comparison
While both are fundamental tonal adjustments, Curves offers more nuanced control.
| Feature | Levels | Curves |
|---|---|---|
| Control Points | Black Point, White Point, Midpoint | Up to 16 flexible Anchor Points |
| Adjustments | Sets black/white points, shifts midtone | Selectively stretches/compresses any tonal range |
| Flexibility | Limited | High, allows for complex non-linear adjustments |
| Usage | Basic tonal range and contrast | Advanced tonal mapping, contrast curves, color adjustments |
Unlike levels however, which only has black, white and midpoint control, a tonal curve is controlled using any number of anchor points (small squares below, up to a total of 16). This allows Curves to create an infinite variety of tonal responses.
Common Uses and Examples
- Increasing Contrast: Creating an "S" shaped curve brightens highlights and darkens shadows, increasing overall contrast.
- Brightening Midtones: Adding a point in the middle and dragging it slightly up.
- Darkening Specific Areas: Adding points to isolate a tonal range and dragging that segment of the curve down.
- Color Correction: Curves can also be adjusted per color channel (Red, Green, Blue) to correct color casts or make creative color shifts. For example, adjusting the Blue channel curve can add warmth (by pulling blue down) or coolness (by pushing blue up) to the image.
By mastering the Curves tool, you gain unparalleled control over the luminosity and color balance of your photographs in Photoshop.
