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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 Levels—brighten and darken the pixel values of your image—but it uses a very different interface ( Figure 3.20 ).


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 after 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 open the Curves dialog box, the curve is straight.

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.

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 looked at before and trace a line up to the curve and then over to the output scale, and you'll see that that tone is lighter ( Figure 3.22 ). Most important, all of the surrounding tones have been adjusted to create a smooth transition.


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 brighten or darken selected portions of your image.

Although the gamma adjustment in Levels offers a great way to change the midtones of an image without affecting the white and black points, there are times when you won't want to adjust all of the midtones equally.

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 improves the contrast of the image by darkening some of the shadows while brightening the highlights. Notice that the black and white points are left untouched, so you don't have to worry about clipping.

The lower point on our curve darkens the shadows, while the upper point brightens the highlights. The overall effect is to give a big contrast boost to the image, without hurting the shadows and highlights. In most cases, adding a slight S-curve edit like this to an image will improve its contrast.

Making selective corrections

The ability to add multiple control points means that you can target Curves to adjust very specific parts of your image. For example, overall Figure 3.25 is well exposed. However, when this image is printed on glossy paper, the already dark house becomes much darker.


Figure 3.25 Though this image is generally well exposed, the barn becomes too dark when the image is printed. With a Curves adjustment, it's possible to lighten the barn while leaving the rest of the image untouched.

Ideally, you want to lighten the house without adjusting any other parts of the image. There are many ways to achieve this type of edit in Photoshop. You could select the house using the Pen or Lasso tool and apply an adjustment to only that selected part of the image, or you could use an adjustment layer with a layer mask—the list goes on and on.

With Curves, though, you can constrain the adjustment to only the house by editing only the part of the curve that contains the tonal values that make up the house.

With the Curves dialog box open, you can click anywhere in your image, and a small circle will appear over the curve to indicate the tonal value of the pixel you clicked. So if you click the barn, you see that its tones live in a very particular part of the lower-left end of the curve ( Figure 3.26 ).


Figure 3.26 With the Curves dialog box open, you can click within your image to see where particular tones lie on the curve.

To adjust only that part of the curve, you first "lock down" the rest of the curve by adding control points. These will serve as anchors to prevent other areas of the curve from being affected by your edit ( Figure 3.27 ).


Figure 3.27 To edit just one part of the curve, you first anchor the curve by placing a few control points.

Now you can simply drag the barn part of the curve up to create your edit ( Figure 3.28 ).


Figure 3.28 With the curve anchored, it's possible to edit just the area that contains the tonal values of the barn. With this edit, the barn brightens while the rest of the image remains the same.

Like Levels, Curves becomes much easier to use with practice. With just a little experimentation, you'll get a better feel for how to drag and shape the curve. Be careful, though—even a slight reshaping of the curve can create a big adjustment to your image. Keep your eyes peeled for posterization (the reduction of tones that can turn some transition areas into splotchy masses of solid color), especially if you're targeting a specific area where there aren't a lot of tones, such as deep shadows or highlights. A sudden or steep change in the curve will almost always yield posterizing. Try to keep the bends in your curve gentle.

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