We can achieve exactly the same effect using the Curves command (Image > Image Adjustments > Curves).
However, using the Curves adjustment layer allows us to modify the image non-destructively by adding a special layer in the Layers palette. For details of the differences between adjustment layers and commands see the discussion for "Making Whites Whiter" earlier in the chapter.
In the Curves dialog box, the horizontal axis represents the input level (or the original color values of the image), while the vertical axis represents the output level (the altered color values). By default, the graph is a straight line because the input and output levels are the same, but these levels change as you alter the shape of the line, as shown in the example below. In this solution, we adjusted the curve to darken the midtones.
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