The Tone Curve is a simple input/output (Transfer) function. Okay, that's more or less a brightness adjustment, right? But to get the most out of the Tone Curve, you have to understand what this is doing to your contrast at the same time. I've found the best way to see how that works is to use a grayscale. This simple graphic has 11 values that step from pure black to pure white in 10% increments. It was created in Photoshop and gives you a very graphic view of how the Tone Curve is pushing your tones around. With a Default Curve (a straight line), the tones in this grayscale are nice and evenly spaced out on the histogram.
Grayscale with Default Tone Curve. An increase in contrast generally means that light tones are going to get lighter, and dark tones are going to get darker, right? Let's see what that looks like. Push the Lights control up to +60, and pull the Darks control down to -60, creating a classic "S curve." With those two adjustments on the curve, you're indeed pushing tones away from the middle, creating greater tonal separation in the middle tones. Greater tonal separation equals greater contrast, right? But, then, look at where the tones on the extreme ends are going. In the very darkest and lightest parts of the image, out at the ends of the histogram, you aren't seeing tonal separation, but rather tonal compression. The tones at the ends of the histogram are being compressed together, creating less contrast.
A classic "S curve" increases contrast through the midtones, and that's probably the most interesting thing about the Tone Curve. When you add contrast in one part of the histogram, you have to give it up somewhere else. This is easy to see in the grayscale, but with a little practice, you'll be able to see it right in the shape of the curve you're creating. Anywhere that you're creating a portion of the curve that's greater than 45 degrees—as you clearly see across the midtones in this example—you're increasing contrast. And anywhere the curve is less than 45 degrees, you're flattening, compressing tones or reducing contrast.
Figures 5, 5-a & 5-b:
A classic "S curve" increases contrast through the midtones. Okay, how do you translate that to an image you're trying to correct? Let's look at the photo of the old woman wearing a scarf in Figure 6. This is a scan from a black-and-white film negative, and I feel that the basic black and white points are good.
Old woman portrait with Default Curve, but without any further correction. The scan just feels a bit flat and dark. So, I know I need to brighten up my midtones and add contrast. Pushing up the Darks slider does indeed brighten all the midtones, but the curve that's created also has a giant flat spot right across the middle, flattening the contrast in those areas.
Figures 6-2a & 6-2b:
Old woman portrait with +60 Darks Curve. On the other hand, an almost identical move on the Lights control also brightens the midtones. This curve creates a steeper line through the midtones, increasing contrast in the vital skin-tone areas, creating a much better correction.