You can find tone curves in various places. Understanding what the graph of a Tone Curve shows will help you to understand some of the image adjustments.
The Tone Curve shows the shadow/highlight relationship between the input (Original) values (on the X axis) and output (Result) values (on the Y axis).
The default is an upward curve from bottom left to top right corner. This represents linear relationship between the original and result - that means no changes.
Examples:
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The Gamma >1.0 shifts the shadows toward lights |
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The Gamma < 1.0 shifts the shadows toward dark |
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Contrast changes both shadows and highlights. The curve becomes more vertical making greater differences in midtones. |
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Linear shift changes the brightness of the image. Here the image is darker because the whole range of input is mapped only in the darker area of output |
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RGB Curves works like half contrast - making contrast highlights or dark, but not both at the same time. |
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Changing the R,G,B curves alone creates different color adjustments. |
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With Levels you can adjust all curves separate in various ways making almost any color corrections. |
As the curve becomes more vertical - the contrast of the image becomes stronger.
As the curve changes more toward vertical - the image becomes less contrasty.
By changing Gamma or Contrast settings you change the shape of the curve(s). This changes the tonal feeling of the image, reduces or enhances midtones etc..