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What are tone curves?

A tone curve is a visual representation of the relationship between the highlights, midtones, and shadows in an image. Tone curves appear in the form of a graph, in which the horizontal X-axis represents the original tonal values (or input), while the vertical Y-axis represents the adjustments made to the original values (or output).

Image by Jim Jag Image by Jim Jag
Tone-curve commands are found in a number of applications, both in Raw processors such as Lightroom and Aperture, and in post-production pixel-editing programs such as Photoshop, Photoshop Elements, and PaintShop Pro.
Tone curves provide a powerful and extremely versatile means of adjusting the entire range of tones present in an image by dragging points on the tone-curve line. Broadly speaking, the lower third of the line represents the shadows, the middle third reflects the midtones, and the upper third stands for the highlights.
By default, the tone curve forms a straight diagonal line (because no adjustments have yet been made, so each input value on the X-axis corresponds exactly to its equivalent on the Y-axis). Adjusting the shape of the curve alters the relationship between the various tones in the image.

Histogram overlay

This example of a default tone curve overlays the image’s histogram—a graph representing the image’s distribution of tonal values. Like the tone curve, the histogram moves along the horizontal X-axis from shadows, to midtones, to highlights.

Midtone brightening

Clicking on the central point of the line and dragging it upward will brighten the entire image, but primarily the midtones. Note that by dragging the curve, you pull up not only that particular segment of the curve, but also all the rest of the curve— which is to say, Curves adjustments are global, affecting the entire image.

Midtone darkening

Clicking on the central point of the line and dragging it down will darken the entire image. Again, it is primarily affecting the midtones, but all other areas are also affected, with the adjustment gradually becoming less significant as you move away from the midtone areas.

The S-Curve

A common curve adjustment is the “S”-shape curve. This is formed by clicking in the highlight region and moving the point up, while at the same time selecting a shadow point and moving it down. The resulting high-contrast image adds greater detail in the midtones, but loses detail in the highlights and shadows, which become brighter and darker, respectively.

Multiple adjustments

Here, a highlight point has been selected to darken the highlights in the top right of the image, while other points have been added to create extra contrast in the rest of the scene.

(Images by Steve Luck.)
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