Adjusting Contrast Using the Tone Curve
Working with the Tone Curve enables you to adjust contrast selectively for different parts of the tonal range. Tone curve corrections shouldn't be applied until after the image has been processed as necessary with the controls in the Basic panel. The tone curve maps the distribution of tonal values in the input image along the x axis to a new distribution of tonal values in the output image along the y axis. The dark end of the range is at the lower left and the light at the upper right (see Figure 7).
A linear tone curve at a 45-degree incline from the lower-left corner to the upper-right corner has no effect on the image; each tone value in the input image is mapped to the identical tone value in the output image. Raising the tone curve above this line maps tone values to a lighter value; lowering it darkens the tonal values.
A tone curve section that's flatter than 45 degrees compresses a range of tone values from the input image to a narrower range in the output image. Some tone values that were distinguishable in the input image become indistinguishable in the output image, and image detail is lost. A tone curve section that's steeper than 45 degrees expands tone values; the differences between tone values become more noticeable and the image contrast is increased.
In Lightroom, the tone curve is constrained so that the curve is always ascending—if you increase the incline of one section of the curve, you'll end up with a decreased incline somewhere else. You'll have to compromise. When using the tone curve, the trick is to increase the contrast in the range where you have the most tonal information. At the same time, try to place the flatter parts of the tone curve in ranges where there's less information in the image (a trough in the histogram) or where a lack of contrast isn't as disturbing or noticeable. A typical tone curve intended to increase midrange contrast starts flat in the lower-left corner (less contrast in the shadows), is steep in the center (more contrast in the midtones), and ends flat in the upper-right corner (less contrast in the highlights), as shown in Figure 8.
For the image at hand, we'll customize the tone curve to increase the contrast selectively in the well-lit wall area of the church, which is the focus of attention in the photo. For this substantial enhancement, it will be worthwhile to sacrifice a little of the contrast in the shadowed area at lower left and in the sky.
- If necessary, expand the Tone Curve panel in the right panel group. In the Navigator, set the Loupe view zoom level to Fit.
- From the Point Curve menu at the bottom right of the Tone Curve panel, choose Linear, Medium Contrast, and Strong Contrast in turn and notice the effect each setting has on the image. You can use any of these tone curve presets as a starting point for your adjustments. For this image, Medium Contrast works best.
- In the tone curve display, note the silhouette of the histogram plotted in the background. This gives you an indication of the tonal ranges where an increase in contrast might be most effective.
- Move the pointer over the tone curve. As you move from left to right, the names of the ranges over which you're moving are displayed below the tone curve grid (see Figure 9).
Whether you use the sliders, enter numeric values, or drag directly in the tone curve grid, the tone curve controls raise or lower the curve by moving the center points of these four ranges: Shadows, Darks, Lights, and Highlights. The overall shape of the tone curve changes to accommodate your adjustments, becoming flatter in one place and steeper in another. The extent to which you can adjust each section of the tone curve is indicated by the gray area that appears when you move the mouse over that section.
- To see which area in the image corresponds to which tonal range, click the Target button in the upper-left corner of the Tone Curve panel and move the pointer over the image in the Loupe view. As you move the pointer over the shadowed area in the lower left of the image, you can see that these tones account for much of the first peak in the histogram around the 25% mark. Most of the tonal values in the blue sky are represented in the second peak in the histogram, just below the 50% input level. The tones in the well-lit church wall are mostly spread between input levels of 60% to 90% (see Figure 10).
Lowering the Lights value in the tone curve should produce the effect we're after: increasing the contrast in the well-lit church wall by steepening the curve for the input values above 60%. The compromise is a flattening of the curve for lower input values—reducing contrast in the sky and shadow areas where a loss of detail is less noticeable.
- Move the tone curve target cursor over the well-lit church wall. When the Lights adjustment control is selected on the Tone Curve, click and drag downward in the image. Release the mouse button when the Lights value is adjusted to -40. (You can adjust the tone curve by dragging directly in the image only when you're using the Tone Curve target mode.)
- Click the Target button in the upper-left corner of the Tone Curve panel again to turn off target mode.
- To compare your image with and without the tone curve adjustment, switch the tone curve adjustment off and on by clicking the On/Off switch icon (circled in Figure 11) at the left side of the Tone Curve panel header. Review the effect in a few different areas of the image at 100% zoom level. You can see how effectively our adjustment enhanced the image detail in the stone wall of the church.
- When you're done, set the zoom level to Fit and make sure that the tone curve adjustment is turned on. The image now looks quite acceptable. To improve it even further, try applying additional sharpening and reduce some of the noise visible in the sky area.