Correcting Tone and Contrast with Curves
If you're using Photoshop 7, CS, or CS2, then you have access to Curves, another type of tone and contrast correction tool. Curves allows you to do the same thing as Levelsbrighten and
Figure 3.20. The Curves dialog box shows an interactive graph with original tones represented by the horizontal grayscale ramp, and adjusted tones represented by the vertical grayscale ramp.
First off, notice that unlike Levels, Curves does not provide you with a histogram display. If you're using Photoshop CS or CS2, then you can use the Histogram palette to watch your image's histogram while you edit.
Next, notice that there are two grayscale ramps: a horizontal one across the bottom, and a vertical one running up the side. The horizontal ramp is the input scale, and it represents the values in your image before the Curves adjustment. The vertical ramp is the output scale, and it represents the values that your image will have
the adjustment. The diagonal line through the center of the box shows the correspondence of the input scale to the output scale. This line is your curve, though when you first
When you open Curves, if you trace a line from a gray tone on the input scale up to the curve, and from there directly across to the output scale, you'll see that the input tone corresponds to an identical output tone. In other words, the curve is not changing the value of the tone ( Figure 3.21 ).
Figure 3.21. Reading the Curves dialog box is very simple. For any particular original, or input tone, just trace a line up to the curve and over to the output graph to see what that tone will be after the Curves edit is applied.
In Photoshop CS/CS2, you access the Curves dialog box by choosing Image > Adjustments > Curves or by pressing Ctrl-M (Windows) or Command-M (Mac). Photoshop Elements does not offer a Curves control.
By reshaping the curve, you can remap the input tones to new output values. To change the shape of the curve, you simply click it and drag. This will add a new control point in addition to the two on the ends. The curve is redrawn to pass through this new point.
For example, if you click the middle of the curve and drag straight up, you will create a new control point and reshape the curve. Now go back to the input tone you
Figure 3.22. Dragging the curve upward causes the original tone to be remapped to a lighter value. All of the tones around that value are remapped to create a smooth transition.
Curves actually does the same thing as Levels. The lower-left control point on the curve is the same thing as the black point in the Levels dialog box. Similarly, the upper-right point on the curve is equivalent to the Levels white point. When you add a point to the middle of the curve, you get a gamma control.
So to use Curves to perform the same correction that you saw in Figure 3.12, you would define a curve that looks like Figure 3.23 .
Figure 3.23. The end points of the curve allow you to change the black and white points of the image, just as the black and white control points in the Levels dialog box do.
The main advantage of Curves over Levels is that whereas Levels lets you adjust only 3 points, Curves lets you add up to 14 control points. By adding more points, you can
Although the gamma adjustment in Levels offers a great way to change the midtones of an image without
For example, Figure 3.24 shows another image that could use more contrast. Although we could increase the contrast by moving the white and black points inward, we would clip both the highlights and the shadows. With Curves, we can leave the white and black points where they are and apply separate adjustments to the middle of the curve.
Figure 3.24. Applying an S-curve to this image