For adjusting image tone, Photoshopís Curves tool offers an amazing amount of control. Of course, thereís also the Brightness/Contrast tool, but because it uniformly affects every pixel, itís best for simple adjustments. And thereís the Levels tool, which allows for adjusting three variables: shadows, midtones, and highlights.
But the Curves tool lets us experiment with up to 255 levels while keeping other values constant. Curves can make a good picture great. Even more fun, it can make a lousy picture look unexpectedly interesting.
Problem images. For overly bright or high-key images, the Curves tool lets you darken areas without losing attractive shadows. For low-key images, it can brighten a problem area without blowing out another. Sometimes the problem is an average-key image, one in which the image tone is, well, monotonous. The Curves tool can bring out lovely contrast in all these compositions.
For this exercise, select a photo that needs a tonal adjustment. Or select one, as we did, that you think has a good balance of tones. You may be surprised to discover how Photoshop can transform a perfectly good photo into one thatís perfectly stunning. Be sure to save the practice image into a new folder so your experiments donít ruin the original.
The Curves Dialog Box
With the image open, go to Image, Adjust, and Curves. Select the Preview checkbox in the lower right; leave the Channel setting at RGB.
The diagonal line. By default, the diagonal line controls values from 0 (shadows) at the bottom left to 255 (highlights) in the upper right. The line is divided into fourths. (If you wish, you can ALT-click to divide the grid into tenths. For this exercise, however, weíll use the default division into quarters.)
Select a point on the line. Now you can click to select the point (or points) on the line that you want to remain constant or fixed. You can select up to 16 fixed points.
If youíre unaccustomed to guessing the intensity of tonal values, left-click the mouse button and drag the pointer around the image. The tonal value of any given pixel is reflected on the diagonal line. And the input reading will reflect the numeric tonal value. Use this to help you select a point on the line.
For example, if you want to keep shadow values fixed, click on the one-quarter point. (The input reading would be 64.) If you want to keep the midtones fixed, click the halfway point, which has an input reading of about 128.
Create Some CurvesNow, use the pointer to pull up on the diagonal line (to increase brightness) or down (to increase shadow). Once you release the mouse button, you leave another fixed point. You cannot remove the fixed point, but you can move it or return it to its original position.
The black square on the curve is the active point. You can adjust the active point by dragging with the mouse or by typing new input and output values.
If you havenít used the Curves tool before, youíll need to experiment to see how adjusting tonal curves can affect a photo. You may get some awful results at first. If so, undo your work and try again. Soon youíll come to understand how this tool can work for you.
An average-key image. For our average-key image, we clicked the one-quarter point. We wanted to boost the intensity of the highlights, so we clicked at about the three-quarter mark and dragged up on the line. By doing so, we also were adjusting the midtones, which looked flat.
Although we created a relatively shallow curve, suddenly the blond strands in our subjectís hair were much brighter. The skin tones, which we thought looked fine, looked much less gray. The small print in the clothing, once lost in shadow, came alive.
Edit channels.Another neat feature of the Curves tool is being able to independently adjust color channels. So if you have an image thatís a bit too pink, you can select the red channel and pull up a bit.
For amateur photographers, Curves is an amazing tool. Not only can it fix up those not-quite-perfectly lit shots, but it can also make a simple snapshot look nearly as flashy as a photo taken by a professional.