The Tone Curve controls offer a new approach to tone curve mapping (using a term called parametetric curves in Lightroom and Camera Raw 4.0), where the tone curve is modified by making slider control adjustments. Tone Curve controls are presented in this way to encourage people to make tone curve adjustments based on descriptive criteria. If you’re used to working with curves in Photoshop, the Lightroom method may appear restrictive at first, but the Tone Curve slider controls in Lightroom may inspire you to create tone curve shapes that are quite unlike the curve shapes you might have applied when adjusting them manually.
The designers recognized that a lot of photographers just didn’t get how to work the curves adjustment in Photoshop. Hopefully, the Tone Curve sliders will make curve adjustments accessible to everyone. One great feature is that you still can manipulate the curve graph directly by clicking a point on the curve and dragging up or down to modify that particular section of the curve. Best of all, however, you also can edit the curve by targeting an area of interest in the actual picture; after you click the Target Adjustment tool, you can click over any part of the image and drag the mouse up or down to make the tones there lighter or darker. When you use this method of tone editing to refine the tones in an image, you won’t even need to look at the Tone Curve panel.
You also can use your keyboard’s arrow keys: The up and down arrows adjust the tone values. (However, note that the left and right arrows are reserved for navigating images in the Filmstrip.) You can turn off the Target Adjustment tool by clicking the button again or by pressing Command-Alt-Shift-N (Mac) or Ctrl-Alt-Shift-N (PC).
The four main slider controls for controlling the tone curve are Highlights, Lights, Darks, and Shadows. The slider controls also provide a shaded preview of the range of shapes an individual Tone Curve slider adjustment will make. In Figure 1, I’m in the process of adjusting the Darks slider. The gray shaded area represents the limits of all possible tone curve shapes I can create with this particular slider in conjunction with the other current slider settings. For those who understand curves, this provides a useful visual reference of how the curve looks. Plus, you can edit it by clicking anywhere on the curve and moving the mouse up or down to make that section of the tone curve lighter or darker.
Figure 1 The Tone Curve panel controls are shown here with an adjustment in progress being made to the Darks. Notice how the histogram in the Histogram panel is mirrored in the curve graph; both are updated as you edit the Tone Curve controls.
The Basic panel is used to apply the main tone adjustments, and it’s important to understand that these are all applied upstream of the tone curve, so the Tone Curve is an image adjustment control that you always apply after making the initial Basic panel adjustments. The layout of the tools in both the Basic and Tone Curve panels is influenced to some degree by the legacy constraints of the Adobe Camera Raw plug-in. For example, the Contrast control in the Basic panel is mainly there to provide an equivalent slider control to the one found in the Camera Raw plug-in. So those people who prefer using the simpler Camera Raw method of adjusting contrast can continue to do so. More importantly, it has been necessary to ensure that settings applied to an image via Camera Raw in Photoshop will be recognized (and made accessible) when that image is opened via the Develop module in Lightroom.
I mention all this background as an explanation for the presence of the Point Curve menu at the bottom of the Tone Curve panel (see Figure 2).
Figure 2 The Point Curve menu offers a choice of three curve presets.
In the early days of Camera Raw, some purists argued that the tone curve should always default to Linear, and if contrast was needed, it was up to the user to edit the curve. Meanwhile, almost every other raw converter program was applying a moderate amount of contrast to the curve by default. The reason for the default application was that photographers liked their pictures to have a more contrasty and film-like look as the standard setting. Consequently, the Adobe Camera Raw plug-in has evolved to offer three choices of curve contrast, with Medium Contrast as the default setting. So the Point Curve menu in the Tone Curve panel is mainly there to match up raw files that have been imported with legacy Camera Raw settings. The Medium Contrast curve applies more of a kick to the shadows to make them slightly darker and lightens the highlights slightly (which you can see by looking at the shape of the curve). The Point Curve is therefore nothing more than a curve shape setting that can be used as a starting point for making further edits to the tone curve, and mainly exists for legacy and compatibility reasons.
The Tone Range Split Points at the bottom of the tone curve allow you to restrict or broaden the range of tones that are affected by the four Tone Curve sliders (see Figure 3). For example, moving the Dark Tone Range Split Point to the right offsets the midpoint between the Shadows and Darks adjustments. Adjusting each of the three Tone Range Split Points enables you to fine-tune the shape of the curve further still. These adjustment sliders are particularly useful for those instances when you’re unable to achieve the exact localized contrast adjustment you want by using the Tone Curve sliders alone.
Figure 3 Tone Range Split Point controls.