Adobe Photoshop Unmasked: The Art and Science of Selections, Layers, and Paths

Using Curves to "Select" Tone and Color

Curves allow you to "select" specific tonal ranges in an image and modify them without affectingor without affecting as muchthe other tonal areas. For example, you can brighten the highlights without affecting the shadows and vice versa. In comparison, Brightness/Contrast either brightens or adds contrast to all the pixels in your image.

Figure 5.2. Luminosity Blend Mode. The original image (example A). A Curves adjustment applied to brighten the image also causes the train to take on a blue cast (example B). Changing the blend mode of the adjustment layer to Luminosity means that only the tone, not the color, is affected (example C).

Curves Explained

Here's a crash course in the Curves dialog box, shown in Figure 5.3 on the next page.

Figure 5.3. The Curves dialog box.

RGB images are measured in levels on a scale of 0-255. The gradient at the bottom of the Curves dialog box indicates which side is white (level 255) and which is black (level 0). You can switch the orientation of black and white by clicking the left triangle on the gradient. Doing so measures your image in ink percentages rather than brightness levels, applicable when editing CMYK images. All screenshots show RGB curves where white is on the right.

Curves begin as a 45-degree line indicating that no changes have been made (the output values equal the input values). An "S" curve adds contrast by brightening the highlights and deepening the shadows. See Figure 5.4.

Figure 5.4. Applying an "S" curve. The original image (example A). The S curve applied to boost brightness and contrast (example B). And the Curve itself (example C).

As well as adjusting the composite RGB curve, you can work on each of the color channels independently by selecting a channel from the Channel pop-up menu at the top of the dialog box.

Levels does the same thing as Curvesselects specific tonal ranges in an imagebut without the same degree of control. With Levels you work with a histogram that displays the number of pixels for each tonal value in the image. To see a histogram while using Curves, open the Histogram palette.

Use These Shortcuts in the Curves Dialog Box

  • Click a point on the curve and drag it up to brighten the image, drag down to darken the image. The brightening or darkening changes the image at the value specified. The effect on neighboring brightness values diminishes the farther those values are from the specified point.

  • To target a specific tone while the Curves dialog box is open, Command/Ctrl-click the image to add a point on the curve precisely at that tone.

  • To remove a point from the curve, simply drag it outside the grid.

  • Option/Alt click the grid to toggle between a 4x4 or a 10x10 square grid.

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