Learning Curves in Photoshop CS2
The Curves dialog box
The Curves dialog (see Figure 1) is one that confuses many new users, but it’s worth getting to grips with. The graph shows how the input values (the original image) translate into the output values (the result of the operation); by subtly tweaking this graph, we can adjust not only the overall brightness and contrast but each of the red, green and blue channels as well.
Figure 1: The Curves dialog box
Using Curves often involves thinking backwards: so if an image is too red, for example, the best fix is often to increase the blue and green components, rather than to reduce the red (which would result in too dark a result). Subtlety is the key: small adjustments are always the best way forward.
- When you open the Curves dialog you see, perhaps surprisingly, not a curve but a straight line. As a default, the input values are the same as the output, so there’s no change.
- By clicking in the center of the line and dragging upwards, we make our first curve. Raising the curve increases the overall brightness of the scene.
- Conversely, clicking in the center and dragging downwards lowers the brightness, producing an image that’s darker overall.
- As well as clicking in the center, we can also adjust the endpoints. Clicking the top right point and dragging down limits the brightest part of the image, reducing contrast.
- If we drag that top right point to the left rather than down, we produce the opposite effect—increasing the contrast of the image. This is a very useful and controllable quick fix.
- By dragging the top and bottom points towards the center, we create a stylized, posterized effect that turns any photograph into more of a graphic object.
- We can also adjust each color individually. Choose the color channel from the pop-up menu: we’ll correct the red cast by adding green to the image.
- To correct our image further, we need to add more blue. Switching to the Blue channel allows us to raise the blue content. But the image is now too washed-out.
- Click once in the center of the RGB curve to "pin" that midpoint; now drag just the top half of the curve to make this S shape, and the result is to increase the overall contrast.
Where to go from here
For more information about adjusting images in Photoshop, see the following:
- Basic photo corrections in Photoshop CS2 from Adobe Photoshop CS2 Classroom in a Book
- Simple adjustment layers by Bruce Fraser and David Blatner
- Adjustment layer basics by Bruce Fraser and David Blatner
