Exposure and Contrast are your first line of attack when you begin fine-tuning a photo’s basic tonal distribution. And just like White Balance, the Basic tone controls are being applied to the raw luminance data before it’s tone mapped into Lightroom’s RGB working space. Exposure changes move the scene’s captured Exposure Values up and down together, preserving their original relationship to each other. This is why Exposure units in Lightroom are measured in stops, just as they are on your camera. Meaning, if you move Exposure up two stops in Lightroom, the relative brightness in the mid-tones will be exactly the same as if you opened up two stops in the camera. Likewise, moving Exposure down two stops in Lightroom will give you mid-tone values that would match those obtained by decreasing exposure in the camera by the same amount. Of course, clipping points change with exposure changes in camera. But for pictures that don’t clip and fit within the camera’s dynamic range, changing Exposure in Lightroom looks identical.
Contrast is easy to understand, but I always like to compare it to changing contrast with the Tone Curve, because that makes it even more graphic. (It helps people understand how the Tone Curve works, too.) Pushing Contrast up is nearly identical to making a basic S-shape with the Tone Curve, pushing Lights up and pulling Darks down. Increasing Contrast pushes tones away from the middle, making values that are already above the 50 percent mark even brighter, and making tones that are less than 50 percent even darker. Negative Contrast values do the opposite, pulling tones in toward the middle.
So, Exposure and Contrast are pretty easy. Highlights, Shadows, Whites and Blacks solve different problems, and are the most frequently misunderstood. The first thing to notice about these four controls is that they all start in the middle. Moving any one of them to the right makes some part of the tonal range brighter, and moving any one of them to the left makes that part of the range darker. But just saying that doesn’t help you know when to use one rather than the other, so perhaps this will help: In general, the Highlights and Shadows controls are used to recover highlight or shadow tones, while Blacks and Whites are used as clipping controls. And this is an important distinction, because all four controls can go either direction. But as I said, in general (meaning probably 95 percent of the time), Highlights will be pulled to the left (negative values) for bringing highlight details down into play, while Shadows will be pushed to the right (positive values) to open up your shadows.
To get a better understanding of how these next controls work on various tones, I recommend making or finding a simple 10- or 20-step grayscale, importing it into Lightroom or Camera Raw and watching the histogram as you push things around (Figure 3).
Even though the step wedge file is already tone mapped (starting out in a gamma 2.2sRGB or Adobe RGB space), in most cases, it will show you exactly what would be happening to those values in RAW, too. The minor difference will be the way clipping behaves, but for most purposes, using a grayscale or RGB file will be a very close approximation of how RAW values act. (I can only guess that maintaining this symmetry between RAW and RGB was no small feat for the Adobe engineers.)